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| </head> |
| <body class="article"> |
| <div id="header"> |
| <h1>Bundle URIs</h1> |
| </div> |
| <div id="content"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Git bundles are files that store a pack-file along with some extra metadata, |
| including a set of refs and a (possibly empty) set of necessary commits. See |
| <a href="../git-bundle.html">git-bundle(1)</a> and <a href="../gitformat-bundle.html">gitformat-bundle(5)</a> for more information.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Bundle URIs are locations where Git can download one or more bundles in |
| order to bootstrap the object database in advance of fetching the remaining |
| objects from a remote.</p> |
| </div> |
| <div class="paragraph"> |
| <p>One goal is to speed up clones and fetches for users with poor network |
| connectivity to the origin server. Another benefit is to allow heavy users, |
| such as CI build farms, to use local resources for the majority of Git data |
| and thereby reducing the load on the origin server.</p> |
| </div> |
| <div class="paragraph"> |
| <p>To enable the bundle URI feature, users can specify a bundle URI using |
| command-line options or the origin server can advertise one or more URIs |
| via a protocol v2 capability.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_design_goals">Design Goals</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The bundle URI standard aims to be flexible enough to satisfy multiple |
| workloads. The bundle provider and the Git client have several choices in |
| how they create and consume bundle URIs.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Bundles can have whatever name the server desires. This name could refer |
| to immutable data by using a hash of the bundle contents. However, this |
| means that a new URI will be needed after every update of the content. |
| This might be acceptable if the server is advertising the URI (and the |
| server is aware of new bundles being generated) but would not be |
| ergonomic for users using the command line option.</p> |
| </li> |
| <li> |
| <p>The bundles could be organized specifically for bootstrapping full |
| clones, but could also be organized with the intention of bootstrapping |
| incremental fetches. The bundle provider must decide on one of several |
| organization schemes to minimize client downloads during incremental |
| fetches, but the Git client can also choose whether to use bundles for |
| either of these operations.</p> |
| </li> |
| <li> |
| <p>The bundle provider can choose to support full clones, partial clones, |
| or both. The client can detect which bundles are appropriate for the |
| repository’s partial clone filter, if any.</p> |
| </li> |
| <li> |
| <p>The bundle provider can use a single bundle (for clones only), or a |
| list of bundles. When using a list of bundles, the provider can specify |
| whether or not the client needs <em>all</em> of the bundle URIs for a full |
| clone, or if <em>any</em> one of the bundle URIs is sufficient. This allows the |
| bundle provider to use different URIs for different geographies.</p> |
| </li> |
| <li> |
| <p>The bundle provider can organize the bundles using heuristics, such as |
| creation tokens, to help the client prevent downloading bundles it does |
| not need. When the bundle provider does not provide these heuristics, |
| the client can use optimizations to minimize how much of the data is |
| downloaded.</p> |
| </li> |
| <li> |
| <p>The bundle provider does not need to be associated with the Git server. |
| The client can choose to use the bundle provider without it being |
| advertised by the Git server.</p> |
| </li> |
| <li> |
| <p>The client can choose to discover bundle providers that are advertised |
| by the Git server. This could happen during <code>git</code> <code>clone</code>, during |
| <code>git</code> <code>fetch</code>, both, or neither. The user can choose which combination |
| works best for them.</p> |
| </li> |
| <li> |
| <p>The client can choose to configure a bundle provider manually at any |
| time. The client can also choose to specify a bundle provider manually |
| as a command-line option to <code>git</code> <code>clone</code>.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Each repository is different and every Git server has different needs. |
| Hopefully the bundle URI feature is flexible enough to satisfy all needs. |
| If not, then the feature can be extended through its versioning mechanism.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_server_requirements">Server requirements</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>To provide a server-side implementation of bundle servers, no other parts |
| of the Git protocol are required. This allows server maintainers to use |
| static content solutions such as CDNs in order to serve the bundle files.</p> |
| </div> |
| <div class="paragraph"> |
| <p>At the current scope of the bundle URI feature, all URIs are expected to |
| be HTTP(S) URLs where content is downloaded to a local file using a <code>GET</code> |
| request to that URL. The server could include authentication requirements |
| to those requests with the aim of triggering the configured credential |
| helper for secure access. (Future extensions could use "file://" URIs or |
| SSH URIs.)</p> |
| </div> |
| <div class="paragraph"> |
| <p>Assuming a <code>200</code> <code>OK</code> response from the server, the content at the URL is |
| inspected. First, Git attempts to parse the file as a bundle file of |
| version 2 or higher. If the file is not a bundle, then the file is parsed |
| as a plain-text file using Git’s config parser. The key-value pairs in |
| that config file are expected to describe a list of bundle URIs. If |
| neither of these parse attempts succeed, then Git will report an error to |
| the user that the bundle URI provided erroneous data.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Any other data provided by the server is considered erroneous.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_bundle_lists">Bundle Lists</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Git server can advertise bundle URIs using a set of <code>key=value</code> pairs. |
| A bundle URI can also serve a plain-text file in the Git config format |
| containing these same <code>key=value</code> pairs. In both cases, we consider this |
| to be a <em>bundle list</em>. The pairs specify information about the bundles |
| that the client can use to make decisions for which bundles to download |
| and which to ignore.</p> |
| </div> |
| <div class="paragraph"> |
| <p>A few keys focus on properties of the list itself.</p> |
| </div> |
| <div class="dlist"> |
| <dl> |
| <dt class="hdlist1">bundle.version</dt> |
| <dd> |
| <p>(Required) This value provides a version number for the bundle |
| list. If a future Git change enables a feature that needs the Git |
| client to react to a new key in the bundle list file, then this version |
| will increment. The only current version number is 1, and if any other |
| value is specified then Git will fail to use this file.</p> |
| </dd> |
| <dt class="hdlist1">bundle.mode</dt> |
| <dd> |
| <p>(Required) This value has one of two values: <code>all</code> and <code>any</code>. When <code>all</code> |
| is specified, then the client should expect to need all of the listed |
| bundle URIs that match their repository’s requirements. When <code>any</code> is |
| specified, then the client should expect that any one of the bundle URIs |
| that match their repository’s requirements will suffice. Typically, the |
| <code>any</code> option is used to list a number of different bundle servers |
| located in different geographies.</p> |
| </dd> |
| <dt class="hdlist1">bundle.heuristic</dt> |
| <dd> |
| <p>If this string-valued key exists, then the bundle list is designed to |
| work well with incremental <code>git</code> <code>fetch</code> commands. The heuristic signals |
| that there are additional keys available for each bundle that help |
| determine which subset of bundles the client should download. The only |
| heuristic currently planned is <code>creationToken</code>.</p> |
| </dd> |
| </dl> |
| </div> |
| <div class="paragraph"> |
| <p>The remaining keys include an <em><id></em> segment which is a server-designated |
| name for each available bundle. The <em><id></em> must contain only alphanumeric |
| and <code>-</code> characters.</p> |
| </div> |
| <div class="dlist"> |
| <dl> |
| <dt class="hdlist1">bundle.<id>.uri</dt> |
| <dd> |
| <p>(Required) This string value is the URI for downloading bundle <em><id></em>. |
| If the URI begins with a protocol (<code>http://</code> or <code>https://</code>) then the URI |
| is absolute. Otherwise, the URI is interpreted as relative to the URI |
| used for the bundle list. If the URI begins with <code>/</code>, then that relative |
| path is relative to the domain name used for the bundle list. (This use |
| of relative paths is intended to make it easier to distribute a set of |
| bundles across a large number of servers or CDNs with different domain |
| names.)</p> |
| </dd> |
| <dt class="hdlist1">bundle.<id>.filter</dt> |
| <dd> |
| <p>This string value represents an object filter that should also appear in |
| the header of this bundle. The server uses this value to differentiate |
| different kinds of bundles from which the client can choose those that |
| match their object filters.</p> |
| </dd> |
| <dt class="hdlist1">bundle.<id>.creationToken</dt> |
| <dd> |
| <p>This value is a nonnegative 64-bit integer used for sorting the bundles |
| list. This is used to download a subset of bundles during a fetch when |
| <code>bundle.heuristic=creationToken</code>.</p> |
| </dd> |
| <dt class="hdlist1">bundle.<id>.location</dt> |
| <dd> |
| <p>This string value advertises a real-world location from where the bundle |
| URI is served. This can be used to present the user with an option for |
| which bundle URI to use or simply as an informative indicator of which |
| bundle URI was selected by Git. This is only valuable when |
| <code>bundle.mode</code> is <code>any</code>.</p> |
| </dd> |
| </dl> |
| </div> |
| <div class="paragraph"> |
| <p>Here is an example bundle list using the Git config format:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle] |
| version = 1 |
| mode = all |
| heuristic = creationToken</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-09-1644442601-daily"] |
| uri = https://bundles.example.com/git/git/2022-02-09-1644442601-daily.bundle |
| creationToken = 1644442601</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-02-1643842562"] |
| uri = https://bundles.example.com/git/git/2022-02-02-1643842562.bundle |
| creationToken = 1643842562</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-09-1644442631-daily-blobless"] |
| uri = 2022-02-09-1644442631-daily-blobless.bundle |
| creationToken = 1644442631 |
| filter = blob:none</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-02-1643842568-blobless"] |
| uri = /git/git/2022-02-02-1643842568-blobless.bundle |
| creationToken = 1643842568 |
| filter = blob:none</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This example uses <code>bundle.mode=all</code> as well as the |
| <code>bundle.</code><em><id></em><code>.creationToken</code> heuristic. It also uses the <code>bundle.</code><em><id></em><code>.filter</code> |
| options to present two parallel sets of bundles: one for full clones and |
| another for blobless partial clones.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Suppose that this bundle list was found at the URI |
| <code>https://bundles.example.com/git/git/</code> and so the two blobless bundles have |
| the following fully-expanded URIs:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>https://bundles.example.com/git/git/2022-02-09-1644442631-daily-blobless.bundle</code></p> |
| </li> |
| <li> |
| <p><code>https://bundles.example.com/git/git/2022-02-02-1643842568-blobless.bundle</code></p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_advertising_bundle_uris">Advertising Bundle URIs</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>If a user knows a bundle URI for the repository they are cloning, then |
| they can specify that URI manually through a command-line option. However, |
| a Git host may want to advertise bundle URIs during the clone operation, |
| helping users unaware of the feature.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The only thing required for this feature is that the server can advertise |
| one or more bundle URIs. This advertisement takes the form of a new |
| protocol v2 capability specifically for discovering bundle URIs.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The client could choose an arbitrary bundle URI as an option <em>or</em> select |
| the URI with best performance by some exploratory checks. It is up to the |
| bundle provider to decide if having multiple URIs is preferable to a |
| single URI that is geodistributed through server-side infrastructure.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_cloning_with_bundle_uris">Cloning with Bundle URIs</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The primary need for bundle URIs is to speed up clones. The Git client |
| will interact with bundle URIs according to the following flow:</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>The user specifies a bundle URI with the <code>--bundle-uri</code> command-line |
| option <em>or</em> the client discovers a bundle list advertised by the |
| Git server.</p> |
| </li> |
| <li> |
| <p>If the downloaded data from a bundle URI is a bundle, then the client |
| inspects the bundle headers to check that the prerequisite commit OIDs |
| are present in the client repository. If some are missing, then the |
| client delays unbundling until other bundles have been unbundled, |
| making those OIDs present. When all required OIDs are present, the |
| client unbundles that data using a refspec. The refspec used is |
| <code>+refs/*:refs/bundles/*</code>. These refs are stored so that later |
| <code>git</code> <code>fetch</code> negotiations can communicate each bundled ref as a <code>have</code>, |
| reducing the size of the fetch over the Git protocol. To allow pruning |
| refs from this ref namespace, Git may introduce a numbered namespace |
| (such as <code>refs/bundles/</code><em><i></em><code>/*</code>) such that stale bundle refs can be |
| deleted.</p> |
| </li> |
| <li> |
| <p>If the file is instead a bundle list, then the client inspects the |
| <code>bundle.mode</code> to see if the list is of the <code>all</code> or <code>any</code> form.</p> |
| <div class="olist loweralpha"> |
| <ol class="loweralpha"> |
| <li> |
| <p>If <code>bundle.mode=all</code>, then the client considers all bundle |
| URIs. The list is reduced based on the <code>bundle.</code><em><id></em><code>.filter</code> options |
| matching the client repository’s partial clone filter. Then, all |
| bundle URIs are requested. If the <code>bundle.</code><em><id></em><code>.creationToken</code> |
| heuristic is provided, then the bundles are downloaded in decreasing |
| order by the creation token, stopping when a bundle has all required |
| OIDs. The bundles can then be unbundled in increasing creation token |
| order. The client stores the latest creation token as a heuristic |
| for avoiding future downloads if the bundle list does not advertise |
| bundles with larger creation tokens.</p> |
| </li> |
| <li> |
| <p>If <code>bundle.mode=any</code>, then the client can choose any one of the |
| bundle URIs to inspect. The client can use a variety of ways to |
| choose among these URIs. The client can also fallback to another URI |
| if the initial choice fails to return a result.</p> |
| </li> |
| </ol> |
| </div> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <p>Note that during a clone we expect that all bundles will be required, and |
| heuristics such as <code>bundle.</code><em><uri></em><code>.creationToken</code> can be used to download |
| bundles in chronological order or in parallel.</p> |
| </div> |
| <div class="paragraph"> |
| <p>If a given bundle URI is a bundle list with a <code>bundle.heuristic</code> |
| value, then the client can choose to store that URI as its chosen bundle |
| URI. The client can then navigate directly to that URI during later <code>git</code> |
| <code>fetch</code> calls.</p> |
| </div> |
| <div class="paragraph"> |
| <p>When downloading bundle URIs, the client can choose to inspect the initial |
| content before committing to downloading the entire content. This may |
| provide enough information to determine if the URI is a bundle list or |
| a bundle. In the case of a bundle, the client may inspect the bundle |
| header to determine that all advertised tips are already in the client |
| repository and cancel the remaining download.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_fetching_with_bundle_uris">Fetching with Bundle URIs</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>When the client fetches new data, it can decide to fetch from bundle |
| servers before fetching from the origin remote. This could be done via a |
| command-line option, but it is more likely useful to use a config value |
| such as the one specified during the clone.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The fetch operation follows the same procedure to download bundles from a |
| bundle list (although we do <em>not</em> want to use parallel downloads here). We |
| expect that the process will end when all prerequisite commit OIDs in a |
| thin bundle are already in the object database.</p> |
| </div> |
| <div class="paragraph"> |
| <p>When using the <code>creationToken</code> heuristic, the client can avoid downloading |
| any bundles if their creation tokens are not larger than the stored |
| creation token. After fetching new bundles, Git updates this local |
| creation token.</p> |
| </div> |
| <div class="paragraph"> |
| <p>If the bundle provider does not provide a heuristic, then the client |
| should attempt to inspect the bundle headers before downloading the full |
| bundle data in case the bundle tips already exist in the client |
| repository.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_error_conditions">Error Conditions</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>If the Git client discovers something unexpected while downloading |
| information according to a bundle URI or the bundle list found at that |
| location, then Git can ignore that data and continue as if it was not |
| given a bundle URI. The remote Git server is the ultimate source of truth, |
| not the bundle URI.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Here are a few example error conditions:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>The client fails to connect with a server at the given URI or a connection |
| is lost without any chance to recover.</p> |
| </li> |
| <li> |
| <p>The client receives a 400-level response (such as <code>404</code> <code>Not</code> <code>Found</code> or |
| <code>401</code> <code>Not</code> <code>Authorized</code>). The client should use the credential helper to |
| find and provide a credential for the URI, but match the semantics of |
| Git’s other HTTP protocols in terms of handling specific 400-level |
| errors.</p> |
| </li> |
| <li> |
| <p>The server reports any other failure response.</p> |
| </li> |
| <li> |
| <p>The client receives data that is not parsable as a bundle or bundle list.</p> |
| </li> |
| <li> |
| <p>A bundle includes a filter that does not match expectations.</p> |
| </li> |
| <li> |
| <p>The client cannot unbundle the bundles because the prerequisite commit OIDs |
| are not in the object database and there are no more bundles to download.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>There are also situations that could be seen as wasteful, but are not |
| error conditions:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>The downloaded bundles contain more information than is requested by |
| the clone or fetch request. A primary example is if the user requests |
| a clone with <code>--single-branch</code> but downloads bundles that store every |
| reachable commit from all <code>refs/heads/*</code> references. This might be |
| initially wasteful, but perhaps these objects will become reachable by |
| a later ref update that the client cares about.</p> |
| </li> |
| <li> |
| <p>A bundle download during a <code>git</code> <code>fetch</code> contains objects already in the |
| object database. This is probably unavoidable if we are using bundles |
| for fetches, since the client will almost always be slightly ahead of |
| the bundle servers after performing its "catch-up" fetch to the remote |
| server. This extra work is most wasteful when the client is fetching |
| much more frequently than the server is computing bundles, such as if |
| the client is using hourly prefetches with background maintenance, but |
| the server is computing bundles weekly. For this reason, the client |
| should not use bundle URIs for fetch unless the server has explicitly |
| recommended it through a <code>bundle.heuristic</code> value.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_example_bundle_provider_organization">Example Bundle Provider organization</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The bundle URI feature is intentionally designed to be flexible to |
| different ways a bundle provider wants to organize the object data. |
| However, it can be helpful to have a complete organization model described |
| here so providers can start from that base.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This example organization is a simplified model of what is used by the |
| GVFS Cache Servers (see section near the end of this document) which have |
| been beneficial in speeding up clones and fetches for very large |
| repositories, although using extra software outside of Git.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The bundle provider deploys servers across multiple geographies. Each |
| server manages its own bundle set. The server can track a number of Git |
| repositories, but provides a bundle list for each based on a pattern. For |
| example, when mirroring a repository at <code>https://</code><em><domain></em><code>/</code><em><org></em><code>/</code><em><repo></em> |
| the bundle server could have its bundle list available at |
| <code>https://</code><em><server-url></em><code>/</code><em><domain></em><code>/</code><em><org></em><code>/</code><em><repo></em>. The origin Git server can |
| list all of these servers under the "any" mode:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle] |
| version = 1 |
| mode = any</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "eastus"] |
| uri = https://eastus.example.com/<domain>/<org>/<repo></pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "europe"] |
| uri = https://europe.example.com/<domain>/<org>/<repo></pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "apac"] |
| uri = https://apac.example.com/<domain>/<org>/<repo></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This "list of lists" is static and only changes if a bundle server is |
| added or removed.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Each bundle server manages its own set of bundles. The initial bundle list |
| contains only a single bundle, containing all of the objects received from |
| cloning the repository from the origin server. The list uses the |
| <code>creationToken</code> heuristic and a <code>creationToken</code> is made for the bundle |
| based on the server’s timestamp.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The bundle server runs regularly-scheduled updates for the bundle list, |
| such as once a day. During this task, the server fetches the latest |
| contents from the origin server and generates a bundle containing the |
| objects reachable from the latest origin refs, but not contained in a |
| previously-computed bundle. This bundle is added to the list, with care |
| that the <code>creationToken</code> is strictly greater than the previous maximum |
| <code>creationToken</code>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>When the bundle list grows too large, say more than 30 bundles, then the |
| oldest "<em>N</em> minus 30" bundles are combined into a single bundle. This |
| bundle’s <code>creationToken</code> is equal to the maximum <code>creationToken</code> among the |
| merged bundles.</p> |
| </div> |
| <div class="paragraph"> |
| <p>An example bundle list is provided here, although it only has two daily |
| bundles and not a full list of 30:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle] |
| version = 1 |
| mode = all |
| heuristic = creationToken</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-13-1644770820-daily"] |
| uri = https://eastus.example.com/<domain>/<org>/<repo>/2022-02-09-1644770820-daily.bundle |
| creationToken = 1644770820</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-09-1644442601-daily"] |
| uri = https://eastus.example.com/<domain>/<org>/<repo>/2022-02-09-1644442601-daily.bundle |
| creationToken = 1644442601</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>[bundle "2022-02-02-1643842562"] |
| uri = https://eastus.example.com/<domain>/<org>/<repo>/2022-02-02-1643842562.bundle |
| creationToken = 1643842562</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>To avoid storing and serving object data in perpetuity despite becoming |
| unreachable in the origin server, this bundle merge can be more careful. |
| Instead of taking an absolute union of the old bundles, instead the bundle |
| can be created by looking at the newer bundles and ensuring that their |
| necessary commits are all available in this merged bundle (or in another |
| one of the newer bundles). This allows "expiring" object data that is not |
| being used by new commits in this window of time. That data could be |
| reintroduced by a later push.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The intention of this data organization has two main goals. First, initial |
| clones of the repository become faster by downloading precomputed object |
| data from a closer source. Second, <code>git</code> <code>fetch</code> commands can be faster, |
| especially if the client has not fetched for a few days. However, if a |
| client does not fetch for 30 days, then the bundle list organization would |
| cause redownloading a large amount of object data.</p> |
| </div> |
| <div class="paragraph"> |
| <p>One way to make this organization more useful to users who fetch frequently |
| is to have more frequent bundle creation. For example, bundles could be |
| created every hour, and then once a day those "hourly" bundles could be |
| merged into a "daily" bundle. The daily bundles are merged into the |
| oldest bundle after 30 days.</p> |
| </div> |
| <div class="paragraph"> |
| <p>It is recommended that this bundle strategy is repeated with the <code>blob:none</code> |
| filter if clients of this repository are expecting to use blobless partial |
| clones. This list of blobless bundles stays in the same list as the full |
| bundles, but uses the <code>bundle.</code><em><id></em><code>.filter</code> key to separate the two groups. |
| For very large repositories, the bundle provider may want to <em>only</em> provide |
| blobless bundles.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_implementation_plan">Implementation Plan</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>This design document is being submitted on its own as an aspirational |
| document, with the goal of implementing all of the mentioned client |
| features over the course of several patch series. Here is a potential |
| outline for submitting these features:</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>Integrate bundle URIs into <code>git</code> <code>clone</code> with a <code>--bundle-uri</code> option. |
| This will include a new <code>git</code> <code>fetch</code> <code>--bundle-uri</code> mode for use as the |
| implementation underneath <code>git</code> <code>clone</code>. The initial version here will |
| expect a single bundle at the given URI.</p> |
| </li> |
| <li> |
| <p>Implement the ability to parse a bundle list from a bundle URI and |
| update the <code>git</code> <code>fetch</code> <code>--bundle-uri</code> logic to properly distinguish |
| between <code>bundle.mode</code> options. Specifically design the feature so |
| that the config format parsing feeds a list of key-value pairs into the |
| bundle list logic.</p> |
| </li> |
| <li> |
| <p>Create the <code>bundle-uri</code> protocol v2 command so Git servers can advertise |
| bundle URIs using the key-value pairs. Plug into the existing key-value |
| input to the bundle list logic. Allow <code>git</code> <code>clone</code> to discover these |
| bundle URIs and bootstrap the client repository from the bundle data. |
| (This choice is an opt-in via a config option and a command-line |
| option.)</p> |
| </li> |
| <li> |
| <p>Allow the client to understand the <code>bundle.heuristic</code> configuration key |
| and the <code>bundle.</code><em><id></em><code>.creationToken</code> heuristic. When <code>git</code> <code>clone</code> |
| discovers a bundle URI with <code>bundle.heuristic</code>, it configures the client |
| repository to check that bundle URI during later <code>git</code> <code>fetch</code> <em><remote></em> |
| commands.</p> |
| </li> |
| <li> |
| <p>Allow clients to discover bundle URIs during <code>git</code> <code>fetch</code> and configure |
| a bundle URI for later fetches if <code>bundle.heuristic</code> is set.</p> |
| </li> |
| <li> |
| <p>Implement the "inspect headers" heuristic to reduce data downloads when |
| the <code>bundle.</code><em><id></em><code>.creationToken</code> heuristic is not available.</p> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <p>As these features are reviewed, this plan might be updated. We also expect |
| that new designs will be discovered and implemented as this feature |
| matures and becomes used in real-world scenarios.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_related_work_packfile_uris">Related Work: Packfile URIs</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Git protocol already has a capability where the Git server can list |
| a set of URLs along with the packfile response when serving a client |
| request. The client is then expected to download the packfiles at those |
| locations in order to have a complete understanding of the response.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This mechanism is used by the Gerrit server (implemented with JGit) and |
| has been effective at reducing CPU load and improving user performance for |
| clones.</p> |
| </div> |
| <div class="paragraph"> |
| <p>A major downside to this mechanism is that the origin server needs to know |
| <em>exactly</em> what is in those packfiles, and the packfiles need to be available |
| to the user for some time after the server has responded. This coupling |
| between the origin and the packfile data is difficult to manage.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Further, this implementation is extremely hard to make work with fetches.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_related_work_gvfs_cache_servers">Related Work: GVFS Cache Servers</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The GVFS Protocol [2] is a set of HTTP endpoints designed independently of |
| the Git project before Git’s partial clone was created. One feature of this |
| protocol is the idea of a "cache server" which can be colocated with build |
| machines or developer offices to transfer Git data without overloading the |
| central server.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The endpoint that VFS for Git is famous for is the <code>GET</code> <code>/gvfs/objects/</code>{oid} |
| endpoint, which allows downloading an object on-demand. This is a critical |
| piece of the filesystem virtualization of that product.</p> |
| </div> |
| <div class="paragraph"> |
| <p>However, a more subtle need is the <code>GET</code> <code>/gvfs/prefetch</code>?lastPackTimestamp=<em><t></em> |
| endpoint. Given an optional timestamp, the cache server responds with a list |
| of precomputed packfiles containing the commits and trees that were introduced |
| in those time intervals.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The cache server computes these "prefetch" packfiles using the following |
| strategy:</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>Every hour, an "hourly" pack is generated with a given timestamp.</p> |
| </li> |
| <li> |
| <p>Nightly, the previous 24 hourly packs are rolled up into a "daily" pack.</p> |
| </li> |
| <li> |
| <p>Nightly, all prefetch packs more than 30 days old are rolled up into |
| one pack.</p> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <p>When a user runs <code>gvfs</code> <code>clone</code> or <code>scalar</code> <code>clone</code> against a repo with cache |
| servers, the client requests all prefetch packfiles, which is at most |
| <code>24</code> <code>+</code> <code>30</code> <code>+</code> <code>1</code> packfiles downloading only commits and trees. The client |
| then follows with a request to the origin server for the references, and |
| attempts to checkout that tip reference. (There is an extra endpoint that |
| helps get all reachable trees from a given commit, in case that commit |
| was not already in a prefetch packfile.)</p> |
| </div> |
| <div class="paragraph"> |
| <p>During a <code>git</code> <code>fetch</code>, a hook requests the prefetch endpoint using the |
| most-recent timestamp from a previously-downloaded prefetch packfile. |
| Only the list of packfiles with later timestamps are downloaded. Most |
| users fetch hourly, so they get at most one hourly prefetch pack. Users |
| whose machines have been off or otherwise have not fetched in over 30 days |
| might redownload all prefetch packfiles. This is rare.</p> |
| </div> |
| <div class="paragraph"> |
| <p>It is important to note that the clients always contact the origin server |
| for the refs advertisement, so the refs are frequently "ahead" of the |
| prefetched pack data. The missing objects are downloaded on-demand using |
| the <code>GET</code> <code>gvfs/objects/</code>{oid} requests, when needed by a command such as |
| <code>git</code> <code>checkout</code> or <code>git</code> <code>log</code>. Some Git optimizations disable checks that |
| would cause these on-demand downloads to be too aggressive.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_see_also">See Also</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>[1] <a href="https://lore.kernel.org/git/RFC-cover-00.13-0000000000-20210805T150534Z-avarab@gmail.com/" class="bare">https://lore.kernel.org/git/RFC-cover-00.13-0000000000-20210805T150534Z-avarab@gmail.com/</a> |
| An earlier RFC for a bundle URI feature.</p> |
| </div> |
| <div class="paragraph"> |
| <p>[2] <a href="https://github.com/microsoft/VFSForGit/blob/master/Protocol.md" class="bare">https://github.com/microsoft/VFSForGit/blob/master/Protocol.md</a> |
| The GVFS Protocol</p> |
| </div> |
| </div> |
| </div> |
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| Last updated 2025-06-20 18:10:42 -0700 |
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