How to choose new master from slaves.?

Hi All,

I have a very basic question about streaming replication feature of Postgres.

Let’s assume I have servers A, B, C, D, & E with postgres installed with streaming replication as follow:

A  à  B ( sync replication )

A  à  C ( async replication )

A  à  D ( async replication )

A  à  E ( async replication )

A is master & rest are slaves.

Now my question is:

Assume replication link between A & B may/may not stable.  If A goes down, how will I choose my new master from slaves? What are criteria to choose new master?

Thanks,

Sacheen Birhade