Erlang -- Appup Cookbook

# 12 Appup Cookbook

This section includes examples of .appup files for typical cases of upgrades/downgrades done in runtime.

###
12.1 Changing a Functional Module

When a functional module has been changed, for example, if a new function has been added or a bug has been corrected, simple code replacement is sufficient, for example:

    {"2",
     [{"1", [{load_module, m}]}],
     [{"1", [{load_module, m}]}]
    }.

###
12.2 Changing a Residence Module

In a system implemented according to the OTP design principles, all processes, except system processes and special processes, reside in one of the behaviours supervisor, gen_server, gen_fsm, gen_statem or gen_event. These belong to the STDLIB application and upgrading/downgrading normally requires an emulator restart.

OTP thus provides no support for changing residence modules except in the case of special processes.

###
12.3 Changing a Callback Module

A callback module is a functional module, and for code extensions simple code replacement is sufficient.

Example: When adding a function to ch3, as described in the example in Release Handling, ch_app.appup looks as follows:

    {"2",
     [{"1", [{load_module, ch3}]}],
     [{"1", [{load_module, ch3}]}]
    }.

OTP also supports changing the internal state of behaviour processes, see Changing Internal State.

###
12.4 Changing Internal State

In this case, simple code replacement is not sufficient. The process must explicitly transform its state using the callback function code_change before switching to the new version of the callback module. Thus, synchronized code replacement is used.

Example: Consider gen_server ch3 from gen_server Behaviour. The internal state is a term Chs representing the available channels. Assume you want to add a counter N, which keeps track of the number of alloc requests so far. This means that the format must be changed to {Chs,N}.

The .appup file can look as follows:

    {"2",
     [{"1", [{update, ch3, {advanced, []}}]}],
     [{"1", [{update, ch3, {advanced, []}}]}]
    }.

The third element of the update instruction is a tuple {advanced,Extra}, which says that the affected processes are to do a state transformation before loading the new version of the module. This is done by the processes calling the callback function code_change (see the gen_server(3) manual page in STDLIB). The term Extra, in this case [], is passed as is to the function:

    -module(ch3).
    ...
    -export([code_change/3]).
    ...
    code_change({down, _Vsn}, {Chs, N}, _Extra) ->
        {ok, Chs};
    code_change(_Vsn, Chs, _Extra) ->
        {ok, {Chs, 0}}.

The first argument is {down,Vsn} if there is a downgrade, or Vsn if there is a upgrade. The term Vsn is fetched from the 'original' version of the module, that is, the version you are upgrading from, or downgrading to.

The version is defined by the module attribute vsn, if any. There is no such attribute in ch3, so in this case the version is the checksum (a huge integer) of the beam file, an uninteresting value, which is ignored.

The other callback functions of ch3 must also be modified and perhaps a new interface function must be added, but this is not shown here.

###
12.5 Module Dependencies

Assume that a module is extended by adding an interface function, as in the example in Release Handling, where a function available/0 is added to ch3.

If a call is added to this function, say in module m1, a runtime error could can occur during release upgrade if the new version of m1 is loaded first and calls ch3:available/0 before the new version of ch3 is loaded.

Thus, ch3 must be loaded before m1, in the upgrade case, and conversely in the downgrade case. m1 is said to be dependent on ch3. In a release handling instruction, this is expressed by the DepMods element:

    {load_module, Module, DepMods}
    {update, Module, {advanced, Extra}, DepMods}

DepMods is a list of modules, on which Module is dependent.

Example: The module m1 in application myapp is dependent on ch3 when upgrading from "1" to "2", or downgrading from "2" to "1":

    myapp.appup:

    {"2",
     [{"1", [{load_module, m1, [ch3]}]}],
     [{"1", [{load_module, m1, [ch3]}]}]
    }.

    ch_app.appup:

    {"2",
     [{"1", [{load_module, ch3}]}],
     [{"1", [{load_module, ch3}]}]
    }.

If instead m1 and ch3 belong to the same application, the .appup file can look as follows:

    {"2",
     [{"1",
       [{load_module, ch3},
        {load_module, m1, [ch3]}]}],
     [{"1",
       [{load_module, ch3},
        {load_module, m1, [ch3]}]}]
    }.

m1 is dependent on ch3 also when downgrading. systools knows the difference between up- and downgrading and generates a correct relup, where ch3 is loaded before m1 when upgrading, but m1 is loaded before ch3 when downgrading.

###
12.6 Changing Code for a Special Process

In this case, simple code replacement is not sufficient. When a new version of a residence module for a special process is loaded, the process must make a fully qualified call to its loop function to switch to the new code. Thus, synchronized code replacement must be used.

Note

The name(s) of the user-defined residence module(s) must be listed in the Modules part of the child specification for the special process. Otherwise the release handler cannot find the process.

Example: Consider the example ch4 in sys and proc_lib. When started by a supervisor, the child specification can look as follows:

    {ch4, {ch4, start_link, []},
     permanent, brutal_kill, worker, [ch4]}

If ch4 is part of the application sp_app and a new version of the module is to be loaded when upgrading from version "1" to "2" of this application, sp_app.appup can look as follows:

    {"2",
     [{"1", [{update, ch4, {advanced, []}}]}],
     [{"1", [{update, ch4, {advanced, []}}]}]
    }.

The update instruction must contain the tuple {advanced,Extra}. The instruction makes the special process call the callback function system_code_change/4, a function the user must implement. The term Extra, in this case [], is passed as is to system_code_change/4:

    -module(ch4).
    ...
    -export([system_code_change/4]).
    ...

    system_code_change(Chs, _Module, _OldVsn, _Extra) ->
        {ok, Chs}.

- The first argument is the internal state State, passed from function sys:handle_system_msg(Request, From, Parent, Module, Deb, State), and called by the special process when a system message is received. In ch4, the internal state is the set of available channels Chs.
- The second argument is the name of the module (ch4).
- The third argument is Vsn or {down,Vsn}, as described for gen_server:code_change/3 in Changing Internal State.

In this case, all arguments but the first are ignored and the function simply returns the internal state again. This is enough if the code only has been extended. If instead the internal state is changed (similar to the example in Changing Internal State), this is done in this function and {ok,Chs2} returned.

###
12.7 Changing a Supervisor

The supervisor behaviour supports changing the internal state, that is, changing the restart strategy and maximum restart frequency properties, as well as changing the existing child specifications.

Child processes can be added or deleted, but this is not handled automatically. Instructions must be given by in the .appup file.

####
Changing Properties

Since the supervisor is to change its internal state, synchronized code replacement is required. However, a special update instruction must be used.

First, the new version of the callback module must be loaded, both in the case of upgrade and downgrade. Then the new return value of init/1 can be checked and the internal state be changed accordingly.

The following upgrade instruction is used for supervisors:

    {update, Module, supervisor}

Example: To change the restart strategy of ch_sup (from Supervisor Behaviour) from one_for_one to one_for_all, change the callback function init/1 in ch_sup.erl:

    -module(ch_sup).
    ...

    init(_Args) ->
        {ok, {#{strategy => one_for_all, ...}, ...}}.

The file ch_app.appup:

    {"2",
     [{"1", [{update, ch_sup, supervisor}]}],
     [{"1", [{update, ch_sup, supervisor}]}]
    }.

####
Changing Child Specifications

The instruction, and thus the .appup file, when changing an existing child specification, is the same as when changing properties as described earlier:

    {"2",
     [{"1", [{update, ch_sup, supervisor}]}],
     [{"1", [{update, ch_sup, supervisor}]}]
    }.

The changes do not affect existing child processes. For example, changing the start function only specifies how the child process is to be restarted, if needed later on.

The id of the child specification cannot be changed.

Changing the Modules field of the child specification can affect the release handling process itself, as this field is used to identify which processes are affected when doing a synchronized code replacement.

####
Adding and Deleting Child Processes

As stated earlier, changing child specifications does not affect existing child processes. New child specifications are automatically added, but not deleted. Child processes are not automatically started or terminated, this must be done using apply instructions.

Example: Assume a new child process m1 is to be added to ch_sup when upgrading ch_app from "1" to "2". This means m1 is to be deleted when downgrading from "2" to "1":

    {"2",
     [{"1",
       [{update, ch_sup, supervisor},
        {apply, {supervisor, restart_child, [ch_sup, m1]}}
       ]}],
     [{"1",
       [{apply, {supervisor, terminate_child, [ch_sup, m1]}},
        {apply, {supervisor, delete_child, [ch_sup, m1]}},
        {update, ch_sup, supervisor}
       ]}]
    }.

The order of the instructions is important.

The supervisor must be registered as ch_sup for the script to work. If the supervisor is not registered, it cannot be accessed directly from the script. Instead a help function that finds the pid of the supervisor and calls supervisor:restart_child, and so on, must be written. This function is then to be called from the script using the apply instruction.

If the module m1 is introduced in version "2" of ch_app, it must also be loaded when upgrading and deleted when downgrading:

    {"2",
     [{"1",
       [{add_module, m1},
        {update, ch_sup, supervisor},
        {apply, {supervisor, restart_child, [ch_sup, m1]}}
       ]}],
     [{"1",
       [{apply, {supervisor, terminate_child, [ch_sup, m1]}},
        {apply, {supervisor, delete_child, [ch_sup, m1]}},
        {update, ch_sup, supervisor},
        {delete_module, m1}
       ]}]
    }.

As stated earlier, the order of the instructions is important. When upgrading, m1 must be loaded, and the supervisor child specification changed, before the new child process can be started. When downgrading, the child process must be terminated before the child specification is changed and the module is deleted.

###
12.8 Adding or Deleting a Module

Example: A new functional module m is added to ch_app:

    {"2",
     [{"1", [{add_module, m}]}],
     [{"1", [{delete_module, m}]}]

###
12.9 Starting or Terminating a Process

In a system structured according to the OTP design principles, any process would be a child process belonging to a supervisor, see Adding and Deleting Child Processes in Changing a Supervisor.

###
12.10 Adding or Removing an Application

When adding or removing an application, no .appup file is needed. When generating relup, the .rel files are compared and the add_application and remove_application instructions are added automatically.

###
12.11 Restarting an Application

Restarting an application is useful when a change is too complicated to be made without restarting the processes, for example, if the supervisor hierarchy has been restructured.

Example: When adding a child m1 to ch_sup, as in Adding and Deleting Child Processes in Changing a Supervisor, an alternative to updating the supervisor is to restart the entire application:

    {"2",
     [{"1", [{restart_application, ch_app}]}],
     [{"1", [{restart_application, ch_app}]}]
    }.

###
12.12 Changing an Application Specification

When installing a release, the application specifications are automatically updated before evaluating the relup script. Thus, no instructions are needed in the .appup file:

    {"2",
     [{"1", []}],
     [{"1", []}]
    }.

###
12.13 Changing Application Configuration

Changing an application configuration by updating the env key in the .app file is an instance of changing an application specification, see the previous section.

Alternatively, application configuration parameters can be added or updated in sys.config.

###
12.14 Changing Included Applications

The release handling instructions for adding, removing, and restarting applications apply to primary applications only. There are no corresponding instructions for included applications. However, since an included application is really a supervision tree with a topmost supervisor, started as a child process to a supervisor in the including application, a relup file can be manually created.

Example: Assume there is a release containing an application prim_app, which have a supervisor prim_sup in its supervision tree.

In a new version of the release, the application ch_app is to be included in prim_app. That is, its topmost supervisor ch_sup is to be started as a child process to prim_sup.

The workflow is as follows:

Step 1) Edit the code for prim_sup:

    init(...) ->
        {ok, {...supervisor flags...,
              [...,
               {ch_sup, {ch_sup,start_link,[]},
                permanent,infinity,supervisor,[ch_sup]},
               ...]}}.

Step 2) Edit the .app file for prim_app:

    {application, prim_app,
     [...,
      {vsn, "2"},
      ...,
      {included_applications, [ch_app]},
      ...
     ]}.

Step 3) Create a new .rel file, including ch_app:

    {release,
     ...,
     [...,
      {prim_app, "2"},
      {ch_app, "1"}]}.

The included application can be started in two ways. This is described in the next two sections.

####
Application Restart

Step 4a) One way to start the included application is to restart the entire prim_app application. Normally, the restart_application instruction in the .appup file for prim_app would be used.

However, if this is done and a relup file is generated, not only would it contain instructions for restarting (that is, removing and adding) prim_app, it would also contain instructions for starting ch_app (and stopping it, in the case of downgrade). This is because ch_app is included in the new .rel file, but not in the old one.

Instead, a correct relup file can be created manually, either from scratch or by editing the generated version. The instructions for starting/stopping ch_app are replaced by instructions for loading/unloading the application:

    {"B",
     [{"A",
       [],
       [{load_object_code,{ch_app,"1",[ch_sup,ch3]}},
        {load_object_code,{prim_app,"2",[prim_app,prim_sup]}},
        point_of_no_return,
        {apply,{application,stop,[prim_app]}},
        {remove,{prim_app,brutal_purge,brutal_purge}},
        {remove,{prim_sup,brutal_purge,brutal_purge}},
        {purge,[prim_app,prim_sup]},
        {load,{prim_app,brutal_purge,brutal_purge}},
        {load,{prim_sup,brutal_purge,brutal_purge}},
        {load,{ch_sup,brutal_purge,brutal_purge}},
        {load,{ch3,brutal_purge,brutal_purge}},
        {apply,{application,load,[ch_app]}},
        {apply,{application,start,[prim_app,permanent]}}]}],
     [{"A",
       [],
       [{load_object_code,{prim_app,"1",[prim_app,prim_sup]}},
        point_of_no_return,
        {apply,{application,stop,[prim_app]}},
        {apply,{application,unload,[ch_app]}},
        {remove,{ch_sup,brutal_purge,brutal_purge}},
        {remove,{ch3,brutal_purge,brutal_purge}},
        {purge,[ch_sup,ch3]},
        {remove,{prim_app,brutal_purge,brutal_purge}},
        {remove,{prim_sup,brutal_purge,brutal_purge}},
        {purge,[prim_app,prim_sup]},
        {load,{prim_app,brutal_purge,brutal_purge}},
        {load,{prim_sup,brutal_purge,brutal_purge}},
        {apply,{application,start,[prim_app,permanent]}}]}]
    }.

####
Supervisor Change

Step 4b) Another way to start the included application (or stop it in the case of downgrade) is by combining instructions for adding and removing child processes to/from prim_sup with instructions for loading/unloading all ch_app code and its application specification.

Again, the relup file is created manually. Either from scratch or by editing a generated version. Load all code for ch_app first, and also load the application specification, before prim_sup is updated. When downgrading, prim_sup is to updated first, before the code for ch_app and its application specification are unloaded.

    {"B",
     [{"A",
       [],
       [{load_object_code,{ch_app,"1",[ch_sup,ch3]}},
        {load_object_code,{prim_app,"2",[prim_sup]}},
        point_of_no_return,
        {load,{ch_sup,brutal_purge,brutal_purge}},
        {load,{ch3,brutal_purge,brutal_purge}},
        {apply,{application,load,[ch_app]}},
        {suspend,[prim_sup]},
        {load,{prim_sup,brutal_purge,brutal_purge}},
        {code_change,up,[{prim_sup,[]}]},
        {resume,[prim_sup]},
        {apply,{supervisor,restart_child,[prim_sup,ch_sup]}}]}],
     [{"A",
       [],
       [{load_object_code,{prim_app,"1",[prim_sup]}},
        point_of_no_return,
        {apply,{supervisor,terminate_child,[prim_sup,ch_sup]}},
        {apply,{supervisor,delete_child,[prim_sup,ch_sup]}},
        {suspend,[prim_sup]},
        {load,{prim_sup,brutal_purge,brutal_purge}},
        {code_change,down,[{prim_sup,[]}]},
        {resume,[prim_sup]},
        {remove,{ch_sup,brutal_purge,brutal_purge}},
        {remove,{ch3,brutal_purge,brutal_purge}},
        {purge,[ch_sup,ch3]},
        {apply,{application,unload,[ch_app]}}]}]
    }.

###
12.15 Changing Non-Erlang Code

Changing code for a program written in another programming language than Erlang, for example, a port program, is application-dependent and OTP provides no special support for it.

Example: When changing code for a port program, assume that the Erlang process controlling the port is a gen_server portc and that the port is opened in the callback function init/1:

    init(...) ->
        ...,
        PortPrg = filename:join(code:priv_dir(App), "portc"),
        Port = open_port({spawn,PortPrg}, [...]),
        ...,
        {ok, #state{port=Port, ...}}.

If the port program is to be updated, the code for the gen_server can be extended with a code_change function, which closes the old port and opens a new port. (If necessary, the gen_server can first request data that must be saved from the port program and pass this data to the new port):

    code_change(_OldVsn, State, port) ->
        State#state.port ! close,
        receive
            {Port,close} ->
                true
        end,
        PortPrg = filename:join(code:priv_dir(App), "portc"),
        Port = open_port({spawn,PortPrg}, [...]),
        {ok, #state{port=Port, ...}}.

Update the application version number in the .app file and write an .appup file:

    ["2",
     [{"1", [{update, portc, {advanced,port}}]}],
     [{"1", [{update, portc, {advanced,port}}]}]
    ].

Ensure that the priv directory, where the C program is located, is included in the new release package:

    1> systools:make_tar("my_release", [{dirs,[priv]}]).
    ...

###
12.16 Emulator Restart and Upgrade

Two upgrade instructions restart the emulator:

- restart_new_emulator

  Intended when ERTS, Kernel, STDLIB, or SASL is upgraded. It is automatically added when the relup file is generated by systools:make_relup/3,4. It is executed before all other upgrade instructions. For more information about this instruction, see restart_new_emulator (Low-Level) in Release Handling Instructions.

- restart_emulator

  Used when a restart of the emulator is required after all other upgrade instructions are executed. For more information about this instruction, see restart_emulator (Low-Level) in Release Handling Instructions.

If an emulator restart is necessary and no upgrade instructions are needed, that is, if the restart itself is enough for the upgraded applications to start running the new versions, a simple relup file can be created manually:

    {"B",
     [{"A",
       [],
       [restart_emulator]}],
     [{"A",
       [],
       [restart_emulator]}]
    }.

In this case, the release handler framework with automatic packing and unpacking of release packages, automatic path updates, and so on, can be used without having to specify .appup files.

###
12.17 Emulator Upgrade From Pre OTP R15

From OTP R15, an emulator upgrade is performed by restarting the emulator with new versions of the core applications (Kernel, STDLIB, and SASL) before loading code and running upgrade instruction for other applications. For this to work, the release to upgrade from must include OTP R15 or later.

For the case where the release to upgrade from includes an earlier emulator version, systools:make_relup creates a backwards compatible relup file. This means that all upgrade instructions are executed before the emulator is restarted. The new application code is therefore loaded into the old emulator. If the new code is compiled with the new emulator, there can be cases where the beam format has changed and beam files cannot be loaded. To overcome this problem, compile the new code with the old emulator.