Watcher vs Watching in storage pkg is confusing. Watcher should be named
as watchStream since it contains a channel as stream to send out events.
Then we can rename watching to watcher, which actually watches on a key
and send watched events through watchStream.
This commits renames watcher to watchStram.
newStore() uses constants for some important parameters
e.g. batchInerval. It is not suitable for other storage users
(e.g. tools/benchmark). This commit decouples the default parameters
from storage creation. etcd should use a newly added function
newDefaultStore() for creating a store with default parameters.
As @jonboulle pointed out at
https://github.com/coreos/etcd/pull/4070/files#r48441847:
> math/rand is unrelated to crypto/rand; the latter reads from /dev/urandom and
> is relying on the kernel's PRNG. Just remove the seed entirely.
Mutex is a variable, which means there needs to be only one mutex
value per scope. We don't need a separate mutex inside store struct,
**if we assume that `TxnPut` and `TxnRange` are called ONLY ONCE
per transaction (between `TxnBegin` and `TxnEnd`)**, as documented.
Having the same variable name as 'index' type
can be confusing and shadowing other variables.
This gives different variable names to 'index' variables.
This adds map operation tests for unsafeAddWatching, which
could have been failed without https://github.com/coreos/etcd/pull/3939.
It tests if unsafeAddWatching is correctly updating synced map.
One watcher includes multiple watchings, and their events are
sent out through one channel. For the received event, user would like to
know which watching it belongs to.
Introduce a watch ID. When watching on some key, user will get a watch
ID. The watch ID is attached to all events that is observed by this
watch.
This is for coreos#3859 switching slice to map for synced watchings.
For a large amount of synced watchings, map implementation performs better.
When putting 1 million watchers on the same key and canceling them one by
one: original implementation takes 9m7.268221091s, while the one with map
takes only 430.531637ms.
The point is to decouple the key-value storage layer and the
event notification layer clearly. It gives the watchableKV the
flexibility to define whatever event structure it wants without
breaking the ondisk format at key-value storage layer.
Changes:
1. change the format of key and value stored in backend
Store KeyValue struct instead of Event struct in backend value for
better abstraction as xiang suggests. And record the corresponded
action in the backend key.
2. Remove word 'event' from functions
This adds build constraints in order to pass memory-map flags to bolt.Option.
If backend package passes syscall.MAP_POPULATE flag, the boltdb does read-ahead
which speeds up entire-database read, which then leads to faster storage
Restore. Benchmark result shows for 4GB, it opens and loads 6x faster in SSD,
and 12x faster in HDD. For 2GB, 1.6x faster with MAP_POPULATE in SSD.
When getting the watched events, it iterate all keys in putm and delm
to generate the events. If we don't delete the key from putm/delm,
it would range on the key that is not actually put or deleted. This is
incorrect.
Fix the panic that happens when single put/delete is watched.
