Observer is a graphical tool for observing the characteristics of Erlang systems. Observer displays system information, application supervisor trees, process information, ETS tables, Mnesia tables and contains a front end for Erlang tracing.

Run Observer from a standalone node to minimize the impact of the system being observed.

Example:

% erl -sname observer -hidden -setcookie MyCookie -run observer

Select the node to observe with menu Nodes. Menu View > Refresh interval controls how often the view is to be updated. The refresh interval is set per viewer so you can have different settings for each viewer. To minimize the system impact, only the active viewer is updated. Other views are updated when activated.

The mouse buttons behave as expected. Use left-click to select objects, right-click to get a menu with the most used options, and double-click to display information about the selected object. In most viewers with many columns, you can change the sort order by left-clicking the column header.

Tab System displays general information about the active Erlang node and its runtime system, such as build configuration, system capabilities, and overall use statistics.

Tab Load Charts displays graphs of the current resource use on the active Erlang node.

Graph Scheduler Utilization shows scheduler use per scheduler, where each scheduler use has a unique color.

Graph Memory Usage shows the total memory use and per memory category use, where each category has a unique color. The categories are as follows:

Graph IO Usage shows the current I/O load on the system.

Tab Memory Allocators displays detailed information of the carrier size and current memory carriers. For details about memory carriers, see module erts_alloc in application ERTS.

Tab Applications presents application information. Select an application in the left list to display its supervisor tree. The right-click options in the tree are as follows:

Tab Processes lists process information in columns. For each process the following information is displayed:

Option Process info opens a detailed information window on the selected process, including the following:

Option Trace Processes adds the selected process identifiers to tab Trace Overview plus the node that the processes reside on.

Option Trace Named Processes adds the registered name of the processes. This can be useful when tracing is done on many nodes, as processes with that name are then traced on all traced nodes.

Tab Table Viewer lists tables. By default, ETS tables are displayed whereas unreadable private ETS tables and tables created by OTP applications are not diplayed. Use menu View to view "system" ETS tables, unreadable ETS tables, or Mnesia tables.

Double-click to view the table content. To view table information, select the table and activate menu View > Table information.

You can use regular expressions and search for objects, and edit or delete them.

Tab Trace Overview handles tracing. Trace by selecting the processes to be traced and how to trace them. You can trace messages, function calls, and events, where events are process-related events such as spawn, exit, and many others.

To trace function calls, you also need to set up trace patterns. Trace patterns select the function calls to be traced. The number of traced function calls can be further reduced with match specifications. Match specifications can also be used to trigger more information in the trace messages.

Processes are added from the Applications or Processes tabs. A special new identifier, meaning all processes spawned after trace start, can be added with button Add 'new' Process.

When adding processes, a window with trace options is displayed. The chosen options are set for the selected processes. Process options can be changed by right-clicking a process.

Processes added by process identifiers add the nodes these processes reside on in the node list. More nodes can be added by clicking button Add Nodes.

If function calls are traced, trace patterns must be added by clicking button Add Trace Pattern. Select a module, function(s), and a match specification. If no functions are selected, all functions in the module are traced.

Trace patterns can also be added for traced messages. This is done in the same way as for traced function calls. Separate trace patterns must be set for sent and received messages.

A few basic match specifications are provided in the tool, and you can provide your own match specifications. The syntax of match specifications is described in the ERTS User's Guide. To simplify the writing of a match specification, they can also be written as fun/1. For details, see module ms_transform in application STDLIB.

Click button Start Trace to start the trace. By default, trace output is written to a new window. Tracing is stopped when the window is closed, or when clicking button Stop Trace. Trace output can be changed with menu Options > Output. The trace settings, including match specifications, can be saved to, or loaded from, a file.

For details about tracing, see module dbg in application Runtime_Tools and in section "Match specifications in Erlang" in ERTS User's Guide and in module ms_transform in application STDLIB.