How to write logs from one service into separate file?

The MonologBundle logs everything using the same handlers for the whole framework. That means if one of your services needs to log to different handlers, you should create your own Logger/Handler and inject that in your service.

This could be an example config (in yaml):

services:
    my_logger:
        class: Symfony\Bridge\Monolog\Logger
        arguments: [soap]
        calls:
            - [pushHandler, [@my_handler]]

    my_handler:
        class: Monolog\Handler\StreamHandler
        # 200 = INFO, see Monolog::Logger for the values of log levels
        arguments: [%kernel.root_dir%/%kernel.environment%.soap.log, 200]

    soap_service:
        class: Your\Soap\Client
        arguments: [@my_logger]

I hope this clarifies it.

Update: as of symfony 2.1, you can also configure which channels receive which handlers, so you could alternatively do something like this:

services:
     soap_service:
         class: Your\Soap\Client
         arguments: [@logger]
         tags:
             - { name: monolog.logger, channel: soap }

Which creates a new soap channel (i.e. logger instance receiving all handlers), then to configure different handlers for this channel:

monolog:
    handlers:
        main:
            type: stream
            path: %kernel.root_dir%/%kernel.environment%.log
            level: error
            channels: [!soap]
        soap:
            type: stream
            path: %kernel.root_dir%/%kernel.environment%.soap.log
            level: info
            channels: [soap]

This means the main handler will receive everything but the soap channel, and the soap handler will receive only the soap channel. You could also remove the channels key on the main handler if you want your main log file to have everything, but also have a copy of only the soap logs separately. This brings a lot of flexibility, and as you see the channels is an array so you can list channels you want, or use the blacklist !name notation to exclude some and include everything else.