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diff --git a/Documentation/technical/bundle-uri.txt b/Documentation/technical/bundle-uri.txt new file mode 100644 index 0000000..91d3a13 --- /dev/null +++ b/Documentation/technical/bundle-uri.txt @@ -0,0 +1,572 @@ +Bundle URIs +=========== + +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 +linkgit:git-bundle[1] and linkgit:gitformat-bundle[5] for more information. + +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. + +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. + +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. + +Design Goals +------------ + +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. + +* 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. + +* 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. + +* 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. + +* 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 _all_ of the bundle URIs for a full + clone, or if _any_ one of the bundle URIs is sufficient. This allows the + bundle provider to use different URIs for different geographies. + +* 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. + +* 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. + +* The client can choose to discover bundle providers that are advertised + by the Git server. This could happen during `git clone`, during + `git fetch`, both, or neither. The user can choose which combination + works best for them. + +* 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 `git clone`. + +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. + +Server requirements +------------------- + +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. + +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 `GET` +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.) + +Assuming a `200 OK` 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. + +Any other data provided by the server is considered erroneous. + +Bundle Lists +------------ + +The Git server can advertise bundle URIs using a set of `key=value` pairs. +A bundle URI can also serve a plain-text file in the Git config format +containing these same `key=value` pairs. In both cases, we consider this +to be a _bundle list_. The pairs specify information about the bundles +that the client can use to make decisions for which bundles to download +and which to ignore. + +A few keys focus on properties of the list itself. + +bundle.version:: + (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. + +bundle.mode:: + (Required) This value has one of two values: `all` and `any`. When `all` + is specified, then the client should expect to need all of the listed + bundle URIs that match their repository's requirements. When `any` is + specified, then the client should expect that any one of the bundle URIs + that match their repository's requirements will suffice. Typically, the + `any` option is used to list a number of different bundle servers + located in different geographies. + +bundle.heuristic:: + If this string-valued key exists, then the bundle list is designed to + work well with incremental `git fetch` 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 `creationToken`. + +The remaining keys include an `<id>` segment which is a server-designated +name for each available bundle. The `<id>` must contain only alphanumeric +and `-` characters. + +bundle.<id>.uri:: + (Required) This string value is the URI for downloading bundle `<id>`. + If the URI begins with a protocol (`http://` or `https://`) 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 `/`, 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.) + +bundle.<id>.filter:: + 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. + +bundle.<id>.creationToken:: + 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 + `bundle.heuristic=creationToken`. + +bundle.<id>.location:: + 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 + `bundle.mode` is `any`. + +Here is an example bundle list using the Git config format: + + [bundle] + version = 1 + mode = all + heuristic = creationToken + + [bundle "2022-02-09-1644442601-daily"] + uri = https://bundles.example.com/git/git/2022-02-09-1644442601-daily.bundle + creationToken = 1644442601 + + [bundle "2022-02-02-1643842562"] + uri = https://bundles.example.com/git/git/2022-02-02-1643842562.bundle + creationToken = 1643842562 + + [bundle "2022-02-09-1644442631-daily-blobless"] + uri = 2022-02-09-1644442631-daily-blobless.bundle + creationToken = 1644442631 + filter = blob:none + + [bundle "2022-02-02-1643842568-blobless"] + uri = /git/git/2022-02-02-1643842568-blobless.bundle + creationToken = 1643842568 + filter = blob:none + +This example uses `bundle.mode=all` as well as the +`bundle.<id>.creationToken` heuristic. It also uses the `bundle.<id>.filter` +options to present two parallel sets of bundles: one for full clones and +another for blobless partial clones. + +Suppose that this bundle list was found at the URI +`https://bundles.example.com/git/git/` and so the two blobless bundles have +the following fully-expanded URIs: + +* `https://bundles.example.com/git/git/2022-02-09-1644442631-daily-blobless.bundle` +* `https://bundles.example.com/git/git/2022-02-02-1643842568-blobless.bundle` + +Advertising Bundle URIs +----------------------- + +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. + +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. + +The client could choose an arbitrary bundle URI as an option _or_ 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. + +Cloning with Bundle URIs +------------------------ + +The primary need for bundle URIs is to speed up clones. The Git client +will interact with bundle URIs according to the following flow: + +1. The user specifies a bundle URI with the `--bundle-uri` command-line + option _or_ the client discovers a bundle list advertised by the + Git server. + +2. 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 default refspec is + `+refs/heads/*:refs/bundles/*`, but this can be configured. These refs + are stored so that later `git fetch` negotiations can communicate each + bundled ref as a `have`, 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 `refs/bundles/<i>/*`) such that + stale bundle refs can be deleted. + +3. If the file is instead a bundle list, then the client inspects the + `bundle.mode` to see if the list is of the `all` or `any` form. + + a. If `bundle.mode=all`, then the client considers all bundle + URIs. The list is reduced based on the `bundle.<id>.filter` options + matching the client repository's partial clone filter. Then, all + bundle URIs are requested. If the `bundle.<id>.creationToken` + 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. + + b. If `bundle.mode=any`, 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. + +Note that during a clone we expect that all bundles will be required, and +heuristics such as `bundle.<uri>.creationToken` can be used to download +bundles in chronological order or in parallel. + +If a given bundle URI is a bundle list with a `bundle.heuristic` +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 `git +fetch` calls. + +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. + +Fetching with Bundle URIs +------------------------- + +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. + +The fetch operation follows the same procedure to download bundles from a +bundle list (although we do _not_ 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. + +When using the `creationToken` 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. + +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. + +Error Conditions +---------------- + +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. + +Here are a few example error conditions: + +* The client fails to connect with a server at the given URI or a connection + is lost without any chance to recover. + +* The client receives a 400-level response (such as `404 Not Found` or + `401 Not Authorized`). 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. + +* The server reports any other failure response. + +* The client receives data that is not parsable as a bundle or bundle list. + +* A bundle includes a filter that does not match expectations. + +* 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. + +There are also situations that could be seen as wasteful, but are not +error conditions: + +* 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 `--single-branch` but downloads bundles that store every + reachable commit from all `refs/heads/*` references. This might be + initially wasteful, but perhaps these objects will become reachable by + a later ref update that the client cares about. + +* A bundle download during a `git fetch` 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 `bundle.heuristic` value. + +Example Bundle Provider organization +------------------------------------ + +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. + +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. + +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 `https://<domain>/<org>/<repo>` +the bundle server could have its bundle list available at +`https://<server-url>/<domain>/<org>/<repo>`. The origin Git server can +list all of these servers under the "any" mode: + + [bundle] + version = 1 + mode = any + + [bundle "eastus"] + uri = https://eastus.example.com/<domain>/<org>/<repo> + + [bundle "europe"] + uri = https://europe.example.com/<domain>/<org>/<repo> + + [bundle "apac"] + uri = https://apac.example.com/<domain>/<org>/<repo> + +This "list of lists" is static and only changes if a bundle server is +added or removed. + +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 +`creationToken` heuristic and a `creationToken` is made for the bundle +based on the server's timestamp. + +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 `creationToken` is strictly greater than the previous maximum +`creationToken`. + +When the bundle list grows too large, say more than 30 bundles, then the +oldest "_N_ minus 30" bundles are combined into a single bundle. This +bundle's `creationToken` is equal to the maximum `creationToken` among the +merged bundles. + +An example bundle list is provided here, although it only has two daily +bundles and not a full list of 30: + + [bundle] + version = 1 + mode = all + heuristic = creationToken + + [bundle "2022-02-13-1644770820-daily"] + uri = https://eastus.example.com/<domain>/<org>/<repo>/2022-02-09-1644770820-daily.bundle + creationToken = 1644770820 + + [bundle "2022-02-09-1644442601-daily"] + uri = https://eastus.example.com/<domain>/<org>/<repo>/2022-02-09-1644442601-daily.bundle + creationToken = 1644442601 + + [bundle "2022-02-02-1643842562"] + uri = https://eastus.example.com/<domain>/<org>/<repo>/2022-02-02-1643842562.bundle + creationToken = 1643842562 + +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. + +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, `git fetch` 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. + +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. + +It is recommended that this bundle strategy is repeated with the `blob:none` +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 `bundle.<id>.filter` key to separate the two groups. +For very large repositories, the bundle provider may want to _only_ provide +blobless bundles. + +Implementation Plan +------------------- + +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: + +1. Integrate bundle URIs into `git clone` with a `--bundle-uri` option. + This will include a new `git fetch --bundle-uri` mode for use as the + implementation underneath `git clone`. The initial version here will + expect a single bundle at the given URI. + +2. Implement the ability to parse a bundle list from a bundle URI and + update the `git fetch --bundle-uri` logic to properly distinguish + between `bundle.mode` options. Specifically design the feature so + that the config format parsing feeds a list of key-value pairs into the + bundle list logic. + +3. Create the `bundle-uri` 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 `git clone` 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.) + +4. Allow the client to understand the `bundle.heuristic` configuration key + and the `bundle.<id>.creationToken` heuristic. When `git clone` + discovers a bundle URI with `bundle.heuristic`, it configures the client + repository to check that bundle URI during later `git fetch <remote>` + commands. + +5. Allow clients to discover bundle URIs during `git fetch` and configure + a bundle URI for later fetches if `bundle.heuristic` is set. + +6. Implement the "inspect headers" heuristic to reduce data downloads when + the `bundle.<id>.creationToken` heuristic is not available. + +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. + +Related Work: Packfile URIs +--------------------------- + +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. + +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. + +A major downside to this mechanism is that the origin server needs to know +_exactly_ 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. + +Further, this implementation is extremely hard to make work with fetches. + +Related Work: GVFS Cache Servers +-------------------------------- + +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. + +The endpoint that VFS for Git is famous for is the `GET /gvfs/objects/{oid}` +endpoint, which allows downloading an object on-demand. This is a critical +piece of the filesystem virtualization of that product. + +However, a more subtle need is the `GET /gvfs/prefetch?lastPackTimestamp=<t>` +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. + +The cache server computes these "prefetch" packfiles using the following +strategy: + +1. Every hour, an "hourly" pack is generated with a given timestamp. +2. Nightly, the previous 24 hourly packs are rolled up into a "daily" pack. +3. Nightly, all prefetch packs more than 30 days old are rolled up into + one pack. + +When a user runs `gvfs clone` or `scalar clone` against a repo with cache +servers, the client requests all prefetch packfiles, which is at most +`24 + 30 + 1` 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.) + +During a `git fetch`, 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. + +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 `GET gvfs/objects/{oid}` requests, when needed by a command such as +`git checkout` or `git log`. Some Git optimizations disable checks that +would cause these on-demand downloads to be too aggressive. + +See Also +-------- + +[1] https://lore.kernel.org/git/RFC-cover-00.13-0000000000-20210805T150534Z-avarab@gmail.com/ + An earlier RFC for a bundle URI feature. + +[2] https://github.com/microsoft/VFSForGit/blob/master/Protocol.md + The GVFS Protocol |