Mercurial > hg
view hgext/clonebundles.py @ 32697:19b9fc40cc51
revlog: skeleton support for version 2 revlogs
There are a number of improvements we want to make to revlogs
that will require a new version - version 2. It is unclear what the
full set of improvements will be or when we'll be done with them.
What I do know is that the process will likely take longer than a
single release, will require input from various stakeholders to
evaluate changes, and will have many contentious debates and
bikeshedding.
It is unrealistic to develop revlog version 2 up front: there
are just too many uncertainties that we won't know until things
are implemented and experiments are run. Some changes will also
be invasive and prone to bit rot, so sitting on dozens of patches
is not practical.
This commit introduces skeleton support for version 2 revlogs in
a way that is flexible and not bound by backwards compatibility
concerns.
An experimental repo requirement for denoting revlog v2 has been
added. The requirement string has a sub-version component to it.
This will allow us to declare multiple requirements in the course
of developing revlog v2. Whenever we change the in-development
revlog v2 format, we can tweak the string, creating a new
requirement and locking out old clients. This will allow us to
make as many backwards incompatible changes and experiments to
revlog v2 as we want. In other words, we can land code and make
meaningful progress towards revlog v2 while still maintaining
extreme format flexibility up until the point we freeze the
format and remove the experimental labels.
To enable the new repo requirement, you must supply an experimental
and undocumented config option. But not just any boolean flag
will do: you need to explicitly use a value that no sane person
should ever type. This is an additional guard against enabling
revlog v2 on an installation it shouldn't be enabled on. The
specific scenario I'm trying to prevent is say a user with a
4.4 client with a frozen format enabling the option but then
downgrading to 4.3 and accidentally creating repos with an
outdated and unsupported repo format. Requiring a "challenge"
string should prevent this.
Because the format is not yet finalized and I don't want to take
any chances, revlog v2's version is currently 0xDEAD. I figure
squatting on a value we're likely never to use as an actual revlog
version to mean "internal testing only" is acceptable. And
"dead" is easily recognized as something meaningful.
There is a bunch of cleanup that is needed before work on revlog
v2 begins in earnest. I plan on doing that work once this patch
is accepted and we're comfortable with the idea of starting down
this path.
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Fri, 19 May 2017 20:29:11 -0700 |
parents | 702af1ad3b18 |
children | d25802b0eef5 |
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# This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. """advertise pre-generated bundles to seed clones "clonebundles" is a server-side extension used to advertise the existence of pre-generated, externally hosted bundle files to clients that are cloning so that cloning can be faster, more reliable, and require less resources on the server. Cloning can be a CPU and I/O intensive operation on servers. Traditionally, the server, in response to a client's request to clone, dynamically generates a bundle containing the entire repository content and sends it to the client. There is no caching on the server and the server will have to redundantly generate the same outgoing bundle in response to each clone request. For servers with large repositories or with high clone volume, the load from clones can make scaling the server challenging and costly. This extension provides server operators the ability to offload potentially expensive clone load to an external service. Here's how it works. 1. A server operator establishes a mechanism for making bundle files available on a hosting service where Mercurial clients can fetch them. 2. A manifest file listing available bundle URLs and some optional metadata is added to the Mercurial repository on the server. 3. A client initiates a clone against a clone bundles aware server. 4. The client sees the server is advertising clone bundles and fetches the manifest listing available bundles. 5. The client filters and sorts the available bundles based on what it supports and prefers. 6. The client downloads and applies an available bundle from the server-specified URL. 7. The client reconnects to the original server and performs the equivalent of :hg:`pull` to retrieve all repository data not in the bundle. (The repository could have been updated between when the bundle was created and when the client started the clone.) Instead of the server generating full repository bundles for every clone request, it generates full bundles once and they are subsequently reused to bootstrap new clones. The server may still transfer data at clone time. However, this is only data that has been added/changed since the bundle was created. For large, established repositories, this can reduce server load for clones to less than 1% of original. To work, this extension requires the following of server operators: * Generating bundle files of repository content (typically periodically, such as once per day). * A file server that clients have network access to and that Python knows how to talk to through its normal URL handling facility (typically an HTTP server). * A process for keeping the bundles manifest in sync with available bundle files. Strictly speaking, using a static file hosting server isn't required: a server operator could use a dynamic service for retrieving bundle data. However, static file hosting services are simple and scalable and should be sufficient for most needs. Bundle files can be generated with the :hg:`bundle` command. Typically :hg:`bundle --all` is used to produce a bundle of the entire repository. :hg:`debugcreatestreamclonebundle` can be used to produce a special *streaming clone bundle*. These are bundle files that are extremely efficient to produce and consume (read: fast). However, they are larger than traditional bundle formats and require that clients support the exact set of repository data store formats in use by the repository that created them. Typically, a newer server can serve data that is compatible with older clients. However, *streaming clone bundles* don't have this guarantee. **Server operators need to be aware that newer versions of Mercurial may produce streaming clone bundles incompatible with older Mercurial versions.** A server operator is responsible for creating a ``.hg/clonebundles.manifest`` file containing the list of available bundle files suitable for seeding clones. If this file does not exist, the repository will not advertise the existence of clone bundles when clients connect. The manifest file contains a newline (\\n) delimited list of entries. Each line in this file defines an available bundle. Lines have the format: <URL> [<key>=<value>[ <key>=<value>]] That is, a URL followed by an optional, space-delimited list of key=value pairs describing additional properties of this bundle. Both keys and values are URI encoded. Keys in UPPERCASE are reserved for use by Mercurial and are defined below. All non-uppercase keys can be used by site installations. An example use for custom properties is to use the *datacenter* attribute to define which data center a file is hosted in. Clients could then prefer a server in the data center closest to them. The following reserved keys are currently defined: BUNDLESPEC A "bundle specification" string that describes the type of the bundle. These are string values that are accepted by the "--type" argument of :hg:`bundle`. The values are parsed in strict mode, which means they must be of the "<compression>-<type>" form. See mercurial.exchange.parsebundlespec() for more details. :hg:`debugbundle --spec` can be used to print the bundle specification string for a bundle file. The output of this command can be used verbatim for the value of ``BUNDLESPEC`` (it is already escaped). Clients will automatically filter out specifications that are unknown or unsupported so they won't attempt to download something that likely won't apply. The actual value doesn't impact client behavior beyond filtering: clients will still sniff the bundle type from the header of downloaded files. **Use of this key is highly recommended**, as it allows clients to easily skip unsupported bundles. If this key is not defined, an old client may attempt to apply a bundle that it is incapable of reading. REQUIRESNI Whether Server Name Indication (SNI) is required to connect to the URL. SNI allows servers to use multiple certificates on the same IP. It is somewhat common in CDNs and other hosting providers. Older Python versions do not support SNI. Defining this attribute enables clients with older Python versions to filter this entry without experiencing an opaque SSL failure at connection time. If this is defined, it is important to advertise a non-SNI fallback URL or clients running old Python releases may not be able to clone with the clonebundles facility. Value should be "true". Manifests can contain multiple entries. Assuming metadata is defined, clients will filter entries from the manifest that they don't support. The remaining entries are optionally sorted by client preferences (``experimental.clonebundleprefers`` config option). The client then attempts to fetch the bundle at the first URL in the remaining list. **Errors when downloading a bundle will fail the entire clone operation: clients do not automatically fall back to a traditional clone.** The reason for this is that if a server is using clone bundles, it is probably doing so because the feature is necessary to help it scale. In other words, there is an assumption that clone load will be offloaded to another service and that the Mercurial server isn't responsible for serving this clone load. If that other service experiences issues and clients start mass falling back to the original Mercurial server, the added clone load could overwhelm the server due to unexpected load and effectively take it offline. Not having clients automatically fall back to cloning from the original server mitigates this scenario. Because there is no automatic Mercurial server fallback on failure of the bundle hosting service, it is important for server operators to view the bundle hosting service as an extension of the Mercurial server in terms of availability and service level agreements: if the bundle hosting service goes down, so does the ability for clients to clone. Note: clients will see a message informing them how to bypass the clone bundles facility when a failure occurs. So server operators should prepare for some people to follow these instructions when a failure occurs, thus driving more load to the original Mercurial server when the bundle hosting service fails. """ from __future__ import absolute_import from mercurial import ( extensions, wireproto, ) testedwith = 'ships-with-hg-core' def capabilities(orig, repo, proto): caps = orig(repo, proto) # Only advertise if a manifest exists. This does add some I/O to requests. # But this should be cheaper than a wasted network round trip due to # missing file. if repo.vfs.exists('clonebundles.manifest'): caps.append('clonebundles') return caps def extsetup(ui): extensions.wrapfunction(wireproto, '_capabilities', capabilities)