upload-pack: add deepen-since to cut shallow repos based on time
[git/git.git] / Documentation / technical / protocol-capabilities.txt
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1Git Protocol Capabilities
2=========================
3
4Servers SHOULD support all capabilities defined in this document.
5
6On the very first line of the initial server response of either
7receive-pack and upload-pack the first reference is followed by
8a NUL byte and then a list of space delimited server capabilities.
9These allow the server to declare what it can and cannot support
10to the client.
11
12Client will then send a space separated list of capabilities it wants
13to be in effect. The client MUST NOT ask for capabilities the server
14did not say it supports.
15
16Server MUST diagnose and abort if capabilities it does not understand
17was sent. Server MUST NOT ignore capabilities that client requested
18and server advertised. As a consequence of these rules, server MUST
19NOT advertise capabilities it does not understand.
20
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21The 'atomic', 'report-status', 'delete-refs', 'quiet', and 'push-cert'
22capabilities are sent and recognized by the receive-pack (push to server)
23process.
b31222cf 24
9354b9a4 25The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
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26by both upload-pack and receive-pack protocols. The 'agent' capability
27may optionally be sent in both protocols.
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28
29All other capabilities are only recognized by the upload-pack (fetch
30from server) process.
31
32multi_ack
33---------
34
35The 'multi_ack' capability allows the server to return "ACK obj-id
36continue" as soon as it finds a commit that it can use as a common
37base, between the client's wants and the client's have set.
38
39By sending this early, the server can potentially head off the client
40from walking any further down that particular branch of the client's
41repository history. The client may still need to walk down other
42branches, sending have lines for those, until the server has a
43complete cut across the DAG, or the client has said "done".
44
45Without multi_ack, a client sends have lines in --date-order until
46the server has found a common base. That means the client will send
47have lines that are already known by the server to be common, because
48they overlap in time with another branch that the server hasn't found
49a common base on yet.
50
51For example suppose the client has commits in caps that the server
52doesn't and the server has commits in lower case that the client
53doesn't, as in the following diagram:
54
55 +---- u ---------------------- x
56 / +----- y
57 / /
58 a -- b -- c -- d -- E -- F
59 \
60 +--- Q -- R -- S
61
62If the client wants x,y and starts out by saying have F,S, the server
63doesn't know what F,S is. Eventually the client says "have d" and
64the server sends "ACK d continue" to let the client know to stop
6a5d0b0a 65walking down that line (so don't send c-b-a), but it's not done yet,
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66it needs a base for x. The client keeps going with S-R-Q, until a
67gets reached, at which point the server has a clear base and it all
68ends.
69
70Without multi_ack the client would have sent that c-b-a chain anyway,
71interleaved with S-R-Q.
72
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73multi_ack_detailed
74------------------
75This is an extension of multi_ack that permits client to better
76understand the server's in-memory state. See pack-protocol.txt,
77section "Packfile Negotiation" for more information.
78
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79no-done
80-------
81This capability should only be used with the smart HTTP protocol. If
82multi_ack_detailed and no-done are both present, then the sender is
83free to immediately send a pack following its first "ACK obj-id ready"
84message.
85
86Without no-done in the smart HTTP protocol, the server session would
87end and the client has to make another trip to send "done" before
88the server can send the pack. no-done removes the last round and
89thus slightly reduces latency.
90
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91thin-pack
92---------
93
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94A thin pack is one with deltas which reference base objects not
95contained within the pack (but are known to exist at the receiving
96end). This can reduce the network traffic significantly, but it
97requires the receiving end to know how to "thicken" these packs by
98adding the missing bases to the pack.
99
100The upload-pack server advertises 'thin-pack' when it can generate
101and send a thin pack. A client requests the 'thin-pack' capability
102when it understands how to "thicken" it, notifying the server that
103it can receive such a pack. A client MUST NOT request the
104'thin-pack' capability if it cannot turn a thin pack into a
105self-contained pack.
106
107Receive-pack, on the other hand, is assumed by default to be able to
108handle thin packs, but can ask the client not to use the feature by
109advertising the 'no-thin' capability. A client MUST NOT send a thin
110pack if the server advertises the 'no-thin' capability.
111
112The reasons for this asymmetry are historical. The receive-pack
113program did not exist until after the invention of thin packs, so
114historically the reference implementation of receive-pack always
115understood thin packs. Adding 'no-thin' later allowed receive-pack
116to disable the feature in a backwards-compatible manner.
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117
118
119side-band, side-band-64k
120------------------------
121
122This capability means that server can send, and client understand multiplexed
123progress reports and error info interleaved with the packfile itself.
124
125These two options are mutually exclusive. A modern client always
126favors 'side-band-64k'.
127
128Either mode indicates that the packfile data will be streamed broken
129up into packets of up to either 1000 bytes in the case of 'side_band',
130or 65520 bytes in the case of 'side_band_64k'. Each packet is made up
131of a leading 4-byte pkt-line length of how much data is in the packet,
132followed by a 1-byte stream code, followed by the actual data.
133
134The stream code can be one of:
135
136 1 - pack data
137 2 - progress messages
138 3 - fatal error message just before stream aborts
139
140The "side-band-64k" capability came about as a way for newer clients
141that can handle much larger packets to request packets that are
142actually crammed nearly full, while maintaining backward compatibility
143for the older clients.
144
145Further, with side-band and its up to 1000-byte messages, it's actually
146999 bytes of payload and 1 byte for the stream code. With side-band-64k,
147same deal, you have up to 65519 bytes of data and 1 byte for the stream
148code.
149
150The client MUST send only maximum of one of "side-band" and "side-
151band-64k". Server MUST diagnose it as an error if client requests
152both.
153
154ofs-delta
155---------
156
5d1e3415 157Server can send, and client understand PACKv2 with delta referring to
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158its base by position in pack rather than by an obj-id. That is, they can
159send/read OBJ_OFS_DELTA (aka type 6) in a packfile.
160
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161agent
162-----
163
164The server may optionally send a capability of the form `agent=X` to
165notify the client that the server is running version `X`. The client may
166optionally return its own agent string by responding with an `agent=Y`
167capability (but it MUST NOT do so if the server did not mention the
168agent capability). The `X` and `Y` strings may contain any printable
169ASCII characters except space (i.e., the byte range 32 < x < 127), and
170are typically of the form "package/version" (e.g., "git/1.8.3.1"). The
171agent strings are purely informative for statistics and debugging
f745acb0 172purposes, and MUST NOT be used to programmatically assume the presence
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173or absence of particular features.
174
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175shallow
176-------
177
178This capability adds "deepen", "shallow" and "unshallow" commands to
179the fetch-pack/upload-pack protocol so clients can request shallow
180clones.
181
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182deepen-since
183------------
184
185This capability adds "deepen-since" command to fetch-pack/upload-pack
186protocol so the client can request shallow clones that are cut at a
187specific time, instead of depth. Internally it's equivalent of doing
188"rev-list --max-age=<timestamp>" on the server side. "deepen-since"
189cannot be used with "deepen".
190
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191no-progress
192-----------
193
194The client was started with "git clone -q" or something, and doesn't
195want that side band 2. Basically the client just says "I do not
196wish to receive stream 2 on sideband, so do not send it to me, and if
197you did, I will drop it on the floor anyway". However, the sideband
198channel 3 is still used for error responses.
199
200include-tag
201-----------
202
203The 'include-tag' capability is about sending annotated tags if we are
204sending objects they point to. If we pack an object to the client, and
205a tag object points exactly at that object, we pack the tag object too.
206In general this allows a client to get all new annotated tags when it
207fetches a branch, in a single network connection.
208
209Clients MAY always send include-tag, hardcoding it into a request when
210the server advertises this capability. The decision for a client to
211request include-tag only has to do with the client's desires for tag
212data, whether or not a server had advertised objects in the
213refs/tags/* namespace.
214
215Servers MUST pack the tags if their referrant is packed and the client
216has requested include-tags.
217
218Clients MUST be prepared for the case where a server has ignored
219include-tag and has not actually sent tags in the pack. In such
220cases the client SHOULD issue a subsequent fetch to acquire the tags
221that include-tag would have otherwise given the client.
222
223The server SHOULD send include-tag, if it supports it, regardless
224of whether or not there are tags available.
225
226report-status
227-------------
228
9a621ad0 229The receive-pack process can receive a 'report-status' capability,
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230which tells it that the client wants a report of what happened after
231a packfile upload and reference update. If the pushing client requests
232this capability, after unpacking and updating references the server
233will respond with whether the packfile unpacked successfully and if
234each reference was updated successfully. If any of those were not
235successful, it will send back an error message. See pack-protocol.txt
236for example messages.
237
238delete-refs
239-----------
240
241If the server sends back the 'delete-refs' capability, it means that
6a5d0b0a 242it is capable of accepting a zero-id value as the target
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243value of a reference update. It is not sent back by the client, it
244simply informs the client that it can be sent zero-id values
245to delete references.
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246
247quiet
248-----
249
250If the receive-pack server advertises the 'quiet' capability, it is
251capable of silencing human-readable progress output which otherwise may
252be shown when processing the received pack. A send-pack client should
253respond with the 'quiet' capability to suppress server-side progress
254reporting if the local progress reporting is also being suppressed
255(e.g., via `push -q`, or if stderr does not go to a tty).
4acbe91a 256
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257atomic
258------
259
260If the server sends the 'atomic' capability it is capable of accepting
261atomic pushes. If the pushing client requests this capability, the server
262will update the refs in one atomic transaction. Either all refs are
263updated or none.
264
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265allow-tip-sha1-in-want
266----------------------
267
268If the upload-pack server advertises this capability, fetch-pack may
269send "want" lines with SHA-1s that exist at the server but are not
270advertised by upload-pack.
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272allow-reachable-sha1-in-want
273----------------------------
274
275If the upload-pack server advertises this capability, fetch-pack may
276send "want" lines with SHA-1s that exist at the server but are not
277advertised by upload-pack.
278
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279push-cert=<nonce>
280-----------------
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281
282The receive-pack server that advertises this capability is willing
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283to accept a signed push certificate, and asks the <nonce> to be
284included in the push certificate. A send-pack client MUST NOT
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285send a push-cert packet unless the receive-pack server advertises
286this capability.