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From "Sylvain Lebresne (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8831) Create a system table to expose prepared statements
Date Thu, 19 Feb 2015 10:54:12 GMT


Sylvain Lebresne commented on CASSANDRA-8831:

As I said, there could be many ways to solve this. So yes, we could technically do that, but
the interesting question is what would be the advantages and inconvenients of doing it compared
to letting clients do it. Perhaps more importantly, saying that nodes aks their neighbors
doesn't say *how* they do it. And while we could use gossip or a custom verb handler for that,
it feels more complex than worth it to me and in both case strongly imply we'd get this in
a major release (which both means that 2.1 is out of question, and that if we don't want to
push that far away in the future it has to make 3.0, and I'm not sure adding more stuff that
absolutely needs to make 3.0 is a very popular idea). So I continue to think that exposing
this through a table is the best way to go: it's easy, it's something user may be interested
in anyway and as a bonus, once we have CASSANDRA-7622, it could be relatively straighforward
to expose more stuff in that table, like the number of times a given statement has been executed.

Once we've done that, we can ask ourselves whether we want to continue letting clients deal
with prepared statements all by themselves or if we prefer making node query their neighbors
table on startup, but imo that question comes next.  And my personal hunch is that it's not
worth bothering: it's true that it would avoid the problem of multiple clients concurrently
checking if statements are prepared on a node, but I'm not sure that problem is such a big
deal in practice, and on the other side, it feels to me that which statement is prepared on
which node is intrinsically something clients decide (for instance, some statement may only
be needed in some DCs, but C* has no way to know that) and so I'm not sure blindy replicating
all statements from neighbors is necessarily what we want.

> Create a system table to expose prepared statements
> ---------------------------------------------------
>                 Key: CASSANDRA-8831
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Sylvain Lebresne
> Because drivers abstract from users the handling of up/down nodes, they have to deal
with the fact that when a node is restarted (or join), it won't know any prepared statement.
Drivers could somewhat ignore that problem and wait for a query to return an error (that the
statement is unknown by the node) to re-prepare the query on that node, but it's relatively
inefficient because every time a node comes back up, you'll get bad latency spikes due to
some queries first failing, then being re-prepared and then only being executed. So instead,
drivers (at least the java driver but I believe others do as well) pro-actively re-prepare
statements when a node comes up. It solves the latency problem, but currently every driver
instance blindly re-prepare all statements, meaning that in a large cluster with many clients
there is a lot of duplication of work (it would be enough for a single client to prepare the
statements) and a bigger than necessary load on the node that started.
> An idea to solve this it to have a (cheap) way for clients to check if some statements
are prepared on the node. There is different options to provide that but what I'd suggest
is to add a system table to expose the (cached) prepared statements because:
> # it's reasonably straightforward to implement: we just add a line to the table when
a statement is prepared and remove it when it's evicted (we already have eviction listeners).
We'd also truncate the table on startup but that's easy enough). We can even switch it to
a "virtual table" if/when CASSANDRA-7622 lands but it's trivial to do with a normal table
in the meantime.
> # it doesn't require a change to the protocol or something like that. It could even be
done in 2.1 if we wish to.
> # exposing prepared statements feels like a genuinely useful information to have (outside
of the problem exposed here that is), if only for debugging/educational purposes.
> The exposed table could look something like:
> {noformat}
> CREATE TABLE system.prepared_statements (
>    keyspace_name text,
>    table_name text,
>    prepared_id blob,
>    query_string text,
>    PRIMARY KEY (keyspace_name, table_name, prepared_id)
> )
> {noformat}

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