annotate hgext/clonebundles.py @ 26762:26f622859288

clonebundles: rewrite documentation

There are a lot of considerations server operators need to know before
deploying clone bundles. They should be documented. So I rewrote the
extension docs to contain this information.

# 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 (experimental)

"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 a
  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` comand. 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.**

The list of requirements printed by :hg:`debugcreatestreamclonebundle` should
be specified in the ``requirements`` parameter of the *bundle specification
string* for the ``BUNDLESPEC`` manifest property described below. e.g.
``BUNDLESPEC=none-packed1;requirements%3Drevlogv1``.

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.

   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.

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.

The following config options influence the behavior of the clone bundles
feature:

ui.clonebundleadvertise
   Whether the server advertises the existence of the clone bundles feature
   to compatible clients that aren't using it.

   When this is enabled (the default), a server will send a message to
   compatible clients performing a traditional clone informing them of the
   available clone bundles feature. Compatible clients are those that support
   bundle2 and are advertising support for the clone bundles feature.

ui.clonebundlefallback
   Whether to automatically fall back to a traditional clone in case of
   clone bundles failure. Defaults to false for reasons described above.

experimental.clonebundles
   Whether the clone bundles feature is enabled on clients. Defaults to true.

experimental.clonebundleprefers
   List of "key=value" properties the client prefers in bundles. Downloaded
   bundle manifests will be sorted by the preferences in this list. e.g.
   the value "BUNDLESPEC=gzip-v1, BUNDLESPEC=bzip2=v1" will prefer a gzipped
   version 1 bundle type then bzip2 version 1 bundle type.

   If not defined, the order in the manifest will be used and the first
   available bundle will be downloaded.
"""

from mercurial.i18n import _
from mercurial.node import nullid
from mercurial import (
    exchange,
    extensions,
    wireproto,
)

testedwith = 'internal'

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.opener.exists('clonebundles.manifest'):
        caps.append('clonebundles')

    return caps

@wireproto.wireprotocommand('clonebundles', '')
def bundles(repo, proto):
    """Server command for returning info for available bundles to seed clones.

    Clients will parse this response and determine what bundle to fetch.

    Other extensions may wrap this command to filter or dynamically emit
    data depending on the request. e.g. you could advertise URLs for
    the closest data center given the client's IP address.
    """
    return repo.opener.tryread('clonebundles.manifest')

@exchange.getbundle2partsgenerator('clonebundlesadvertise', 0)
def advertiseclonebundlespart(bundler, repo, source, bundlecaps=None,
                              b2caps=None, heads=None, common=None,
                              cbattempted=None, **kwargs):
    """Inserts an output part to advertise clone bundles availability."""
    # Allow server operators to disable this behavior.
    # # experimental config: ui.clonebundleadvertise
    if not repo.ui.configbool('ui', 'clonebundleadvertise', True):
        return

    # Only advertise if a manifest is present.
    if not repo.opener.exists('clonebundles.manifest'):
        return

    # And when changegroup data is requested.
    if not kwargs.get('cg', True):
        return

    # And when the client supports clone bundles.
    if cbattempted is None:
        return

    # And when the client didn't attempt a clone bundle as part of this pull.
    if cbattempted:
        return

    # And when a full clone is requested.
    # Note: client should not send "cbattempted" for regular pulls. This check
    # is defense in depth.
    if common and common != [nullid]:
        return

    msg = _('this server supports the experimental "clone bundles" feature '
            'that should enable faster and more reliable cloning\n'
            'help test it by setting the "experimental.clonebundles" config '
            'flag to "true"')

    bundler.newpart('output', data=msg)

def extsetup(ui):
    extensions.wrapfunction(wireproto, '_capabilities', capabilities)