By default, HAproxy configuration can not be changed without breaking a
connection with the client :)

Dmitry Vasilyev
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

On Fri, 2015-09-18 at 12:27 +1000, James Sewell wrote:
> Hello all,
> I have recently been working with PostgreSQL and HAProxy to provide
> seamless load balancing to a group of database servers. This on it's
> own isn't a hard thing: I have an implementation finished and am now
> thinking about the best way to bring it to a production ready state
> which could be used by others, and used in load-balancers other than
> HAProxy with minimal config changes.
> My initial requirements were:
> Given a group of PostgreSQL servers check each x seconds and:
> Allow read/write access only to the master server (via IPA / portA)
> Disallow access if there are multiple master servers
> Allow read access to all servers (via IPB / portB) as long as the
> following holds:
> They are attached to the current master server via streaming
> replication (or they are the current master server)
> They can currently contact the master server (safest option, disallow
> all access when master-less)
> They are in the same timeline as the master server (do I need this
> check?)
> The master server reports that they have less than x bytes lag
> HAProxy can talk to PostgreSQL for a health check via TCP or PSQL
> (connection check only). Neither of these allow the logic above -
> therefore this logic has to be hosted outside of HAProxy. This might
> change in the future if HAProxy gets the ability to send SQL
> statements (like an F5 can).
> Today the best way to provide this information to  HAProxy (and many
> other load balancers, application frameworks, proxies, monitoring
> solutions) is via HTTP, where HTTP 200 is pass the check and HTTP 503
> is fail the check (and don't load balance to this node). In my case I
> have a script which accepts HTTP requests to /read to check if this
> node is available for read only and /write which checks if this node
> is available for read/writes.
> The options as I see them are:
> Implement a script / small app which connects to PostgreSQL and
> executes these checks
> Implemented and proven today at many independent sites
> Should it run on HAProxy server or PSQL server? 
> Integrated HTTP server  or x.inetd script?
> Platform independence?
> What if it dies?
> Implement a custom PostgreSQL BGworker which provides this
> information over HTTP
> No outside of PostgreSQL config needed
> No reliance on another daemon / interface being up
> libmicrohttpd or similar should help with  platform independence
> Security / acceptance by community?
> Only newer versions of PostgreSQL
> Spend the time working on getting SQL checks into HAProxy
> What about other platforms which only support HTTP?
> I think all of the options would benefit from a PSQL extension which
> does the following:
> Encapsulates the check logic (easier to upgrade, manipulate)
> Stores historic check data for a number of hours / days / months
> Stores defaults (override via HTTP could be possible for things like
> lag)
> Does anyone else have any thoughts on this topic? 
> Eventually many cool features could flow out of this kind of work:
> Integration with High Availability products - I have this working
> with EnterpriseDB EFM now.
> Locate the current master using the HA product 
> more than one master doesn't cause loss of service as long as HA
> state is sane
> Locate all clustered standby servers using the HA product
> if a standby is removed from the HA cluster, it is removed from load
> balancing
> if a standby is not part of the cluster, it is removed from load
> balancing (even if it is part of streaming replication)
> HTTP replication status requests which facilitate dynamically
> managing HAProxy (or other) PostgreSQL server pools
> Add a node to streaming replication, it automatically shows up in the
> pool and starts being checked to see if it can service reads
> Great for cloud scale out
> Allocation of additional load balancer groups based on some criteria
> (?), for example
> read/write (as above)
> read only (as above)
> data warehouse (reporting reads only)
> DR (replica with no reads or writes - until it becomes a master)
> Keen to hear comments.
> Cheers,
> James Sewell,
> Solutions Architect 
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