+++
title = "Subscriptions"
weight = 20
+++

An Algo exists to observe changes in the market and respond to them. When the Algo is [launched](1/server/ref/initialization/#launch-launch-attached) it uses IAlgoServices to subscribe to the events it needs to operate.  Each subscribe call is paired with a matching unsubscribe call, which takes the same argument to unsubscribe.

# Book

Subscribe to book updates for the specified security call use subscribe_book and unsubscribe book.
```cpp
// Subscribe to book updates for the specified security
void IAlgoServices::subscribe_book(SecurityKey);
void IAlgoServices::unsubscribe_book(SecurityKey);

// You implement
virtual void on_book(SecurityKey, Book const &) override;
```
Get the `SecurityKey` of the current security using:
```cpp
services.subscribe_book(SecurityKey(algo_services().security().id()));
```
If your Algo needs to subscribe to other securities, their keys must be passed in as parameters.


To get the details of a security use
```cpp
api::Security const* IBeforeLaunchAlgoServices::lookup_security(SecurityKey k) const = 0;
```

# Timer
To handle time-based logic, an Algo can subscribe to a common timer event that is invoked once every 10 milliseconds.  Use the TimeStamp class from `bts_chrono.h` to efficiently handle time calculations.
```cpp
// Subscribe to a periodic 10ms timer
void IAlgoServices::subscribe_timer();
// You implement
virtual void on_timer() override;
```

# Ticker
The status callback is called when the market for this security opens, closes, or has some other state transition.
```cpp
// Subscribe to trade ticker
void IAlgoServices::subscribe_ticker(SecurityKey);
// You implement
void on_ticker(SecurityKey, TradeVector const&) override;
```

# Trading Status

The status callback is called when the market for this security opens, closes, or has some other state transition.
```cpp
// Subscribe to security trading status
void IAlgoServices::subscribe_status(SecurityKey);
// You implement
virtual void on_status(SecurityKey, co2::TradingStatus) override;
```

# Child Orders

When an Algo launches a child order, it is subscribed to the changes on that child automatically.  If the Algo unsubscribes from those changes, it can re-subscribe using this method.

```cpp
// Subscribe to child order updates for the specified child order.
void IAlgoServices::subscribe_child_change(OrderKey);
// You implement
virtual void on_child_change(OrderKey, ChildState const &) override;
```