view tests/test-batching.py @ 40326:fed697fa1734

sqlitestore: file storage backend using SQLite This commit provides an extension which uses SQLite to store file data (as opposed to revlogs). As the inline documentation describes, there are still several aspects to the extension that are incomplete. But it's a start. The extension does support basic clone, checkout, and commit workflows, which makes it suitable for simple use cases. One notable missing feature is support for "bundlerepos." This is probably responsible for the most test failures when the extension is activated as part of the test suite. All revision data is stored in SQLite. Data is stored as zstd compressed chunks (default if zstd is available), zlib compressed chunks (default if zstd is not available), or raw chunks (if configured or if a compressed delta is not smaller than the raw delta). This makes things very similar to revlogs. Unlike revlogs, the extension doesn't yet enforce a limit on delta chain length. This is an obvious limitation and should be addressed. This is somewhat mitigated by the use of zstd, which is much faster than zlib to decompress. There is a dedicated table for storing deltas. Deltas are stored by the SHA-1 hash of their uncompressed content. The "fileindex" table has columns that reference the delta for each revision and the base delta that delta should be applied against. A recursive SQL query is used to resolve the delta chain along with the delta data. By storing deltas by hash, we are able to de-duplicate delta storage! With revlogs, the same deltas in different revlogs would result in duplicate storage of that delta. In this scheme, inserting the duplicate delta is a no-op and delta chains simply reference the existing delta. When initially implementing this extension, I did not have content-indexed deltas and deltas could be duplicated across files (just like revlogs). When I implemented content-indexed deltas, the size of the SQLite database for a full clone of mozilla-unified dropped: before: 2,554,261,504 bytes after: 2,488,754,176 bytes Surprisingly, this is still larger than the bytes size of revlog files: revlog files: 2,104,861,230 bytes du -b: 2,254,381,614 I would have expected storage to be smaller since we're not limiting delta chain length and since we're using zstd instead of zlib. I suspect the SQLite indexes and per-column overhead account for the bulk of the differences. (Keep in mind that revlog uses a 64-byte packed struct for revision index data and deltas are stored without padding. Aside from the 12 unused bytes in the 32 byte node field, revlogs are pretty efficient.) Another source of overhead is file name storage. With revlogs, file names are stored in the filesystem. But with SQLite, we need to store file names in the database. This is roughly equivalent to the size of the fncache file, which for the mozilla-unified repository is ~34MB. Since the SQLite database isn't append-only and since delta chains can reference any delta, this opens some interesting possibilities. For example, we could store deltas in reverse, such that fulltexts are stored for newer revisions and deltas are applied to reconstruct older revisions. This is likely a more optimal storage strategy for version control, as new data tends to be more frequently accessed than old data. We would obviously need wire protocol support for transferring revision data from newest to oldest. And we would probably need some kind of mechanism for "re-encoding" stores. But it should be doable. This extension is very much experimental quality. There are a handful of features that don't work. It probably isn't suitable for day-to-day use. But it could be used in limited cases (e.g. read-only checkouts like in CI). And it is also a good proving ground for alternate storage backends. As we continue to define interfaces for all things storage, it will be useful to have a viable alternate storage backend to see how things shake out in practice. test-storage.py passes on Python 2 and introduces no new test failures on Python 3. Having the storage-level unit tests has proved to be insanely useful when developing this extension. Those tests caught numerous bugs during development and I'm convinced this style of testing is the way forward for ensuring alternate storage backends work as intended. Of course, test coverage isn't close to what it needs to be. But it is a start. And what coverage we have gives me confidence that basic store functionality is implemented properly. Differential Revision: https://phab.mercurial-scm.org/D4928
author Gregory Szorc <gregory.szorc@gmail.com>
date Tue, 09 Oct 2018 08:50:13 -0700
parents 33a6eee08db2
children b81ca9a3f4e4
line wrap: on
line source

# test-batching.py - tests for transparent command batching
#
# Copyright 2011 Peter Arrenbrecht <peter@arrenbrecht.ch>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from __future__ import absolute_import, print_function

import contextlib

from mercurial import (
    localrepo,
    wireprotov1peer,

)

# equivalent of repo.repository
class thing(object):
    def hello(self):
        return "Ready."

# equivalent of localrepo.localrepository
class localthing(thing):
    def foo(self, one, two=None):
        if one:
            return "%s and %s" % (one, two,)
        return "Nope"
    def bar(self, b, a):
        return "%s und %s" % (b, a,)
    def greet(self, name=None):
        return "Hello, %s" % name

    @contextlib.contextmanager
    def commandexecutor(self):
        e = localrepo.localcommandexecutor(self)
        try:
            yield e
        finally:
            e.close()

# usage of "thing" interface
def use(it):

    # Direct call to base method shared between client and server.
    print(it.hello())

    # Direct calls to proxied methods. They cause individual roundtrips.
    print(it.foo("Un", two="Deux"))
    print(it.bar("Eins", "Zwei"))

    # Batched call to a couple of proxied methods.

    with it.commandexecutor() as e:
        ffoo = e.callcommand('foo', {'one': 'One', 'two': 'Two'})
        fbar = e.callcommand('bar', {'b': 'Eins', 'a': 'Zwei'})
        fbar2 = e.callcommand('bar', {'b': 'Uno', 'a': 'Due'})

    print(ffoo.result())
    print(fbar.result())
    print(fbar2.result())

# local usage
mylocal = localthing()
print()
print("== Local")
use(mylocal)

# demo remoting; mimicks what wireproto and HTTP/SSH do

# shared

def escapearg(plain):
    return (plain
            .replace(':', '::')
            .replace(',', ':,')
            .replace(';', ':;')
            .replace('=', ':='))
def unescapearg(escaped):
    return (escaped
            .replace(':=', '=')
            .replace(':;', ';')
            .replace(':,', ',')
            .replace('::', ':'))

# server side

# equivalent of wireproto's global functions
class server(object):
    def __init__(self, local):
        self.local = local
    def _call(self, name, args):
        args = dict(arg.split('=', 1) for arg in args)
        return getattr(self, name)(**args)
    def perform(self, req):
        print("REQ:", req)
        name, args = req.split('?', 1)
        args = args.split('&')
        vals = dict(arg.split('=', 1) for arg in args)
        res = getattr(self, name)(**vals)
        print("  ->", res)
        return res
    def batch(self, cmds):
        res = []
        for pair in cmds.split(';'):
            name, args = pair.split(':', 1)
            vals = {}
            for a in args.split(','):
                if a:
                    n, v = a.split('=')
                    vals[n] = unescapearg(v)
            res.append(escapearg(getattr(self, name)(**vals)))
        return ';'.join(res)
    def foo(self, one, two):
        return mangle(self.local.foo(unmangle(one), unmangle(two)))
    def bar(self, b, a):
        return mangle(self.local.bar(unmangle(b), unmangle(a)))
    def greet(self, name):
        return mangle(self.local.greet(unmangle(name)))
myserver = server(mylocal)

# local side

# equivalent of wireproto.encode/decodelist, that is, type-specific marshalling
# here we just transform the strings a bit to check we're properly en-/decoding
def mangle(s):
    return ''.join(chr(ord(c) + 1) for c in s)
def unmangle(s):
    return ''.join(chr(ord(c) - 1) for c in s)

# equivalent of wireproto.wirerepository and something like http's wire format
class remotething(thing):
    def __init__(self, server):
        self.server = server
    def _submitone(self, name, args):
        req = name + '?' + '&'.join(['%s=%s' % (n, v) for n, v in args])
        return self.server.perform(req)
    def _submitbatch(self, cmds):
        req = []
        for name, args in cmds:
            args = ','.join(n + '=' + escapearg(v) for n, v in args)
            req.append(name + ':' + args)
        req = ';'.join(req)
        res = self._submitone('batch', [('cmds', req,)])
        for r in res.split(';'):
            yield r

    @contextlib.contextmanager
    def commandexecutor(self):
        e = wireprotov1peer.peerexecutor(self)
        try:
            yield e
        finally:
            e.close()

    @wireprotov1peer.batchable
    def foo(self, one, two=None):
        encargs = [('one', mangle(one),), ('two', mangle(two),)]
        encresref = wireprotov1peer.future()
        yield encargs, encresref
        yield unmangle(encresref.value)

    @wireprotov1peer.batchable
    def bar(self, b, a):
        encresref = wireprotov1peer.future()
        yield [('b', mangle(b),), ('a', mangle(a),)], encresref
        yield unmangle(encresref.value)

    # greet is coded directly. It therefore does not support batching. If it
    # does appear in a batch, the batch is split around greet, and the call to
    # greet is done in its own roundtrip.
    def greet(self, name=None):
        return unmangle(self._submitone('greet', [('name', mangle(name),)]))

# demo remote usage

myproxy = remotething(myserver)
print()
print("== Remote")
use(myproxy)