Load Balance

Hello,
I am having a concept question about how the Read/Write process transaction
works within the DB network structure.
Assume I have an environment of one master DB with three slaves running
Slony-I replication server.
As you all know Slony is a Single master to multiple slaves replication
system supporting cascading and slave promotion. Master DB can do both read
and write, but the slaves can only do read and for reporting purpose.

Here is the question of concept that I don't understand, if you have answer
or provide me a link to referral is appreciated.

Q: For application Developer write codes to accessing the DB server to
getting/modify datas. Since there is only one server that can handle both
Read/Write which is the Master and slave is only to retrive data.
How should the developer know which DB server should they connecting to for
either to read or to write? If developer is using the same fixed connection
strings to a fix IP to Master server for Read/Write and another connection
string to a fix IP to (for example) my SLAVE1 server to retrive datas.
How and what would happen if my SLAVE1 failed or due to maintaince that I
need to replace it?

Also what would happen for Master DB if issues such as failover or switch
over that I need to replacing the master server due to failure or for
maintaince, or to promote one of the slave to master?

I just need answer that tells me how PostgreSQL and Slony would react if
such issues happens. Answer can simply just to provide me a web link for me
to read further. Thanks!