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Clarifies text in schema extension example #135

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Clarifies text in schema extension example
Dec 14, 2020
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less lies
adriancole Dec 14, 2020
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32 changes: 26 additions & 6 deletions v2-0-RC3/doc/05SchemaExtensionMechanism.md
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Expand Up @@ -122,13 +122,18 @@ If the *received version is less than the decoder's version* (that is, the produ

## Message schema extension example

Initial version of a message schema
The following example walks through changes to a schema to reinforce how schema
versioning works.

### Initial schema

Here's the initial example schema:
```xml
<messageSchema package="FIXBinaryTest" byteOrder="littleEndian">
<types>
<type name="int8" primitiveType="int8"/>
</types>

<types>
<type name="int8" primitiveType="int8"/>
</types>

<message name="FIX Binary Message1" id="1" blockLength="4">
<field name="Field1" id="1" type="int8" semanticType="int"/>
Expand All @@ -137,7 +142,13 @@ Initial version of a message schema
</messageSchema>
```

Second version - a new message is added
### Adding elements to the schema

Below adds new elements to the schema. Upon any update we must increment the
`version` attribute. When there is no `version` attribute, the version is zero.
Hence, our new version is one. This version applies to all change.

This update chooses to document updates with the `sinceVersion` attribute.

```xml
<messageSchema package="FIXBinaryTest" byteOrder="littleEndian"
Expand All @@ -161,7 +172,9 @@ Second version - a new message is added
</messageSchema>
```

Third version - a field is added
### Adding a field to a message

This update changes a message from the initial schema and adds a type.

```xml
<messageSchema package="FIXBinaryTest" byteOrder="littleEndian"
Expand All @@ -187,3 +200,10 @@ Third version - a field is added
</message>
</messageSchema>
```

Notice field addition effects only the version of the field: We don't update
`sinceVersion` on a message when we add a field to it.

*Note:* `sinceVersion` is not required, and has no default interpretation. In
other words, "FIX Binary Message1" cannot be assumed as since version zero. To
clarify this, consider adding `sinceVersion` on initial schema elements.