To simplify the semantics of blocking for a group, this commit changes
the implementation to better match the description we provide of
conusmer groups: blocking for > will make the consumer waiting for new
elements in the group. However blocking for any other ID will always
serve the local history of the consumer.
However it must be noted that the > ID is actually an alias for the
special ID ms/seq of UINT64_MAX,UINT64_MAX.
Now a MAXLEN of 0 really does what it means: it will create a zero
entries stream. This is useful in order to make sure that the behavior
is identical to XTRIM, that must be able to reduce the stream to zero
elements when MAXLEN is given.
Also now MAXLEN with a count < 0 will return an error.
Currently it does not look it's sensible to generate events for streams
consumer groups modification, being them metadata, however at least for
key-level events, like the creation or removal of a consumer group, I
added a few events here and there. Later we can evaluate if it makes
sense to add more. From the POV instead of WAIT (in Redis transaciton)
and signaling the key as modified, it looks like that the transaction
should not fail when a stream is modified, so no calls are made in
consumer groups related functions to signalModifiedKey().
Now that we have SETID, the inetrnals of consumer groups should be able
to handle the case of the same message delivered multiple times just
as a side effect of calling XREADGROUP. Normally this should never
happen but if the admin manually "XGROUP SETID mykey mygroup 0",
messages will get re-delivered to clients waiting for the ">" special
ID. The consumer groups internals were not able to handle the case of a
message re-delivered in this circumstances that was already assigned to
another owner.
This should be more than enough, even if in case of partial IDs that are
not found, we send all the IDs to the slave/AOF, but this is definitely
a corner case without bad effects if not some wasted space.
XINFO is mainly an observability command that will be used more by
humans than computers, and even when used by computers it will be a very
low traffic command. For this reason the format was changed in order to
have field names. They'll consume some bandwidth and CPU cycles, but in
this context this is much better than having to understand what the
numbers in the output array are.
After checking with the community via Twitter (here:
https://twitter.com/antirez/status/915130876861788161) the verdict was to
use ":". However I later realized, after users lamented the fact that
it's hard to copy IDs just with double click, that this was the reason
why I moved to "." in the first instance. Fortunately "-", that was the
other option with most votes, also gets selected with double click on
most terminal applications on Linux and MacOS.
So my reasoning was:
1) We can't retain "." because it's actually confusing to newcomers, it
looks like a floating number, people may be tricked into thinking they
can order IDs numerically as floats.
2) Moving to a double-click-to-select format is much better. People will
work with such IDs for long time when coding / debugging. Why making now
a choice that will impact this for the next years?
The only other viable option was "-", and that's what I did. Thanks.
The core of this change is the implementation of stream trimming, and
the resulting MAXLEN option of XADD as a trivial result of having
trimming functionalities. MAXLEN already works but in order to be more
efficient listpack GC should be implemented, currently marked as a TODO
item inside the comments.
Listpack max size is a tradeoff between space and time. A 2k max entry
puts the memory usage approximately at a similar order of magnitude (5
million entries went from 96 to 120 MB), but the range queries speed
doubled (because there are half entries to scan in the average case).
Lower values could be considered, or maybe this parameter should be
made tunable.
We used to have the master ID stored at the start of the listpack,
however using the key directly makes more sense in order to create a
space efficient representation: anyway the key at the radix tree is very
unlikely to change because of how the stream is implemented. Moreover on
nodes merging, to rewrite the merged listpacks is anyway the most
sensible operation, and we can use the iterator and the append-to-stream
function in order to avoid re-implementing the code needed for merging.
This commit also adds two items at the start of the listpack: the
number of valid items inside the listpack, and the number of items
marked as deleted. This means that there is no need to scan a listpack
in order to understand if it's a good candidate for garbage collection,
if the ration between valid/deleted items triggers the GC.
The approach used is to set a fixed header at the start of every
listpack blob (that contains many entries). The header contains a
"master" ID and fields, that are initially just obtained from the first
entry inserted in the listpack, so that the first enty is always well
compressed. Later every new entry is checked against these fields, and
if it matches, the SAMEFIELD flag is set in the entry so that we know to
just use the master entry flags. The IDs are always delta-encoded
against the first entry. This approach avoids cascading effects in which
entries are encoded depending on the previous entries, in order to avoid
complexity and rewritings of the data when data is removed in the middle
(which is a planned feature).
XADD was suboptimal in the first incarnation of the command, not being
able to accept an ID (very useufl for replication), nor options for
having capped streams.
The keyspace notification for streams was not implemented.
A client may lose a lot of time between invocations of blocking XREAD,
for example because it is processing the messages or for any other
cause. When it returns back, it may provide a low enough message ID that
the server will block to send an unreasonable number of messages in a
single call. For this reason we set a COUNT when the client is blocked
with XREAD calls, even if no COUNT is given. This is arbitrarily set to
1000 because it's enough to avoid slowing down the reception of many
messages, but low enough to avoid to block.
With lists we need to signal only on key creation, but streams can
provide data to clients listening at every new item added.
To make this slightly more efficient we now track different classes of
blocked clients to avoid signaling keys when there is nobody listening.
A typical case is when the stream is used as a time series DB and
accessed only by range with XRANGE.
