Editing SyncML (section)

==Internals==

SyncML works by exchanging [[Command-line interface|commands]], which can be requests and responses. As an example:

* the [[mobile phone]] sends an <code>Alert</code> command for signaling the wish to begin a refresh-only [[synchronization]]
* the [[computer]] responds with a <code>Status</code> command for accepting the request
* the phone sends one or more <code>Sync</code> command containing an Add sub-command for each item (e.g., [[phonebook]] entry); if the number of entries is large, it does not include the <Final/> tag;
* in the latter case, the computer requests to continue with an appropriate <code>Alert</code> message, and the mobile sends another chunk of items; otherwise, the computer confirms it received all data with a <code>Status</code> command

Commands (<code>Alert</code>, <code>Sync</code>, <code>Status</code>, etc.) are grouped into messages. Each message and each of its commands has an identifier, so that the pair (MsgID, CmdID) uniquely determines a command. Responses like <code>Status</code> commands include the pair identifying the command they are responding to.

Before commands, messages contain a header specifying various data regarding the [[Database transaction|transaction]]. An example message containing the <code>Alert</code> command for begin a refresh synchronization, like in the previous example, is:

<syntaxhighlight lang="xml">
<?xml version="1.0"?>
<!DOCTYPE SyncML PUBLIC "-//SYNCML//DTD SyncML 1.2//EN" "http://www.openmobilealliance.org/tech/DTD/OMA-TS-SyncML_RepPro_DTD-V1_2.dtd">
<SyncML xmlns="SYNCML:SYNCML1.2">
 <SyncHdr>
  <VerDTD>1.1</VerDTD>
  <VerProto>SyncML/1.1</VerProto>
  <SessionID>1</SessionID>
  <MsgID>1</MsgID>
  <Target><LocURI>PC Suite</LocURI></Target>
  <Source><LocURI>IMEI:3405623856456</LocURI></Source>
  <Meta><MaxMsgSize xmlns="syncml:metinf">8000</MaxMsgSize></Meta>
 </SyncHdr>

 <SyncBody>
  <Alert>
   <CmdID>1</CmdID>
   <Data>203</Data>   <!-- 203 = mobile signals a refresh from it to computer -->
   <Item>
    <Target><LocURI>Events</LocURI></Target>
    <Source><LocURI>/telecom/cal.vcs</LocURI></Source>
    <Meta><Anchor xmlns="syncml:metinf"><Last>42</Last><Next>42</Next></Anchor></Meta>
   </Item>
  </Alert>

  <Final/>
 </SyncBody>
</SyncML>
</syntaxhighlight>

The response from the [[computer]] could be an [[XML]] document like (comments added
for the sake of explanation):

<syntaxhighlight lang="xml">
<?xml version="1.0"?>
<!DOCTYPE SyncML PUBLIC "-//SYNCML//DTD SyncML 1.2//EN" "http://www.openmobilealliance.org/tech/DTD/OMA-TS-SyncML_RepPro_DTD-V1_2.dtd">
<SyncML>
 <SyncHdr>
  <VerDTD>1.1</VerDTD>
  <VerProto>SyncML/1.1</VerProto>
  <SessionID>1</SessionID>
  <MsgID>1</MsgID>
  <Target><LocURI>IMEI:3405623856456</LocURI></Target>
  <Source><LocURI>PC Suite</LocURI></Source>
 </SyncHdr>

 <SyncBody>

  <!-- accept the header of the last message from the client -->
  <Status>
   <CmdID>1</CmdID>
   <MsgRef>1</MsgRef>
   <CmdRef>0</CmdRef>	<!-- 0 = header of the message -->
   <Cmd>SyncHdr</Cmd>
   <TargetRef>PC Suite</TargetRef>
   <SourceRef>IMEI:3405623856456</SourceRef>
   <Data>200</Data>	<!-- 200 = ok, accepted -->
  </Status>

  <!-- accept the request of the mobile for a sync -->
  <Status>
   <CmdID>2</CmdID>	<!-- this is command #2 -->
   <MsgRef>1</MsgRef>
   <CmdRef>1</CmdRef>	<!-- it respond to command msg=1,cmd=1 -->
   <Cmd>Alert</Cmd>
   <TargetRef>Events</TargetRef>
   <SourceRef>/telecom/cal.vcs</SourceRef>
   <Meta><Anchor xmlns="syncml:metinf"><Next>0</Next><Last>0</Last></Anchor></Meta>
   <Data>200</Data>	<!-- 200 = ok, accepted -->
  </Status>

  <Final/>
 </SyncBody>
</SyncML>
</syntaxhighlight>

The transaction then proceeds with a message from the mobile containing the <code>Sync</code> command, and so on.

This example is a refresh where the mobile sends all its data to the computer and nothing in the other way around. Different codes in the initial <code>Alert</code> command can be used to initiate other kinds of synchronizations. For example, in a "[[Two-way communication|two-way]] sync", only the changes from the last [[synchronization]] are sent to the [[computer]], which does the same.

The <code>Last</code> and <code>Next</code> tags are used to keep track of a possible loss of sync. <code>Last</code> represents the time of the last operation of synchronization, as measured by each device. For example, a [[Mobile phone|mobile]] may use progressive numbers (<code>1</code>, <code>2</code>, <code>3</code>, ...) to represent time, while the computer uses strings like "<code>20140112T213401Z</code>". <code>Next</code> is the current time in the same representation. This latter data is stored and then compared with <code>Last</code> in the next synchronization. Any difference indicates a loss of sync. Appropriate actions involving sending all data can be then taken to put the devices back in sync.

Anchors are only used to detect a loss of sync; they do not indicate which data is to be sent. Apart from the loss-of-synchronization situation, in a normal (non-refresh) synchronization, each device sends a [[Logbook|log]] of changes since the last synchronization.