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https://issues.apache.org/jira/browse/HBASE-8372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13639313#comment-13639313
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Jonathan Hsieh commented on HBASE-8372:
---------------------------------------
bq. Can you clarify the meaning of 'a new mutableConf' ? Once mutableConf is
materialized inside CompoundConfiguration, the list of ImmutableConfigMap's are
no longer needed.
I believe this is false. Let me spend the time to explicitly illustrate the
different approaches with a pseudo code unit test.
{code}
Conf hbase = <from hbase-site.xml/hbasedefault.xml>.
Conf htd = <from htabledescription of table t>
Conf hri = <from hregioninfo of region r in table t>
Conf compound = new CompoundConfiguration;
compound.add(hbase);
compound.add(htd);
compound.add(hri);
compound.set("key", "value");
assertEquals("value", compound.get("key")); // passes
Map<..> map = <new map that doesn't overlap any values>;
compound.add(map); // trigger freeze
hbase.set("hbasekey", "hbasevalue");
assertEquals("hbasevalue", compound.get("hbasekey")); // should pass. If the
triggered freeze did deep copy, it would fail because there is no reference to
the higher level's table
hri.setProp("...", "blah");
assertEquals("blah", compound.get("...")); // should pass. If the triggered
freeze did deep copy, it would fail because there is no reference to the higher
level's table
{code}
If we did a deep copy, and eliminated the list of ImmutableConfigMaps, we'd
have a copy of each configuration value, essentially taking up an entry per
property in each of the levels instead of a single pointer to the previous
level's map. This is a common pattern used to manage hierarchical scope.
----
bq. In patch v5, each time freezeMutableConf() is called, ImmutableConfWrapper
would keep a reference to mutableConf. This leads to more memory consumption.
Only if there is a mutableConf present. If I do multiple add* in a row, no new
mutableConf will be added.
I'm surprised I need to be this explicit. I've numbered the relevant lines in
the code.
{code}
+ /**
+ * If set has been called, it will create a mutableConf. This converts the
mutableConf to an
+ * immutable one and resets it to allow a new mutable conf. This is used
when a new map or
+ * conf is added to the compound configuration to preserve proper override
semantics.
+ */
+ void freezeMutableConf() {
1:+ if (mutableConf == null) {
2:+ // do nothing if there is no current mutableConf
3:+ return;
4:+ }
5:+
6:+ this.configs.add(0, new ImmutableConfWrapper(mutableConf));
7:+ mutableConf = null;
8:+ }
+
{code}
# Let's say there has never been a set call. mutableConf is null. At line 1,
we see this is null, and then as the comment says in line 2, we return at line
3.
# Let's say set has been called and mutableConf is not null. At line 1 we see
that mutableConf is not null. We go to line 6 and then wrap and include the
mutable conf to the configs maps. Then at line 7 we null out the mutableConf
#* If set is not called and freeze is called again, mutableConf will be null --
and we proceed via option 1 and exit at line 3. No new memory allocations.
#* If set is called and then freeze is called again, mutableConf will be
non-null -- and we proceed via option 2 and exit at line 7. One new memory
allocation with a pointer to the old mutableConf.
> Provide mutability to CompoundConfiguration
> -------------------------------------------
>
> Key: HBASE-8372
> URL: https://issues.apache.org/jira/browse/HBASE-8372
> Project: HBase
> Issue Type: New Feature
> Reporter: Ted Yu
> Assignee: Jonathan Hsieh
> Attachments: hbase-8372.patch, hbase-8372.v2.patch,
> hbase-8372.v3.patch, hbase-8372.v4.patch, hbase-8372.v5.patch
>
>
> In discussion of HBASE-8347, it was proposed that CompoundConfiguration
> should support mutability.
> This can be done by consolidating ImmutableConfigMap's on first modification
> to CompoundConfiguration.
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