Resolve Oracle prompt failed to connect to database instance:ora-12519 error

Oracle database installed on the Ali Cloud, one day received the following alerts:
Name=atabasename
Type=database Instance
Host=izw2519p6r9uzeee
Metric=status
Timestamp=nov, 2014 4:16:37 AM CST
Severity=critical
message=failed to connect to database Instance:ora-12519:tns:no appropriate service handler found (DBD Error:ociservera Ttach).
Rule name=database availability and Critical States
Rule Owner=sysman

The reason for this problem is that the number of processes exceeds the limit and is queried by the following methods:

# su-oracle

$ sqlplus/as sysdba;

To view the current number of connections:

Sql> Select COUNT (*) from v$process;

View the current number of connections to 299

To view the maximum number of connections allowed for a database:

Sql> Select value from v$parameter where name = ' processes ';

View the maximum number of connections allowed is 300

It seems that the number of connections is not enough to make the following modifications:
Modify the maximum number of connections to 500:

Sql> alter system set processes = Scope = SPFile;

To restart the database:

sql> shutdown immediate;
Sql> startup;